diff --color-moved=zebra: fix alternate coloring

b0a2ba4776 ("diff --color-moved=zebra: be stricter with color
alternation", 2018-11-23) sought to avoid using the alternate colors
unless there are two adjacent moved blocks of the same
sign. Unfortunately it contains two bugs that prevented it from fixing
the problem properly. Firstly `last_symbol` is reset at the start of
each iteration of the loop losing the symbol of the last line and
secondly when deciding whether to use the alternate color it should be
checking if the current line is the same sign of the last line, not a
different sign. The combination of the two errors means that we still
use the alternate color when we should do but we also use it when we
shouldn't. This is most noticable when using
--color-moved-ws=allow-indentation-change with hunks like

-this line gets indented
+    this line gets indented

where the post image is colored with newMovedAlternate rather than
newMoved. While this does not matter much, the next commit will change
the coloring to be correct in this case, so lets fix the bug here to
make it clear why the output is changing and add a regression test.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1176,6 +1176,7 @@ static void mark_color_as_moved(struct diff_options *o,
 	struct moved_block *pmb = NULL; /* potentially moved blocks */
 	int pmb_nr = 0, pmb_alloc = 0;
 	int n, flipped_block = 0, block_length = 0;
+	enum diff_symbol last_symbol = 0;
 
 
 	for (n = 0; n < o->emitted_symbols->nr; n++) {
@@ -1183,7 +1184,6 @@ static void mark_color_as_moved(struct diff_options *o,
 		struct moved_entry *key;
 		struct moved_entry *match = NULL;
 		struct emitted_diff_symbol *l = &o->emitted_symbols->buf[n];
-		enum diff_symbol last_symbol = 0;
 
 		switch (l->s) {
 		case DIFF_SYMBOL_PLUS:
@@ -1251,7 +1251,7 @@ static void mark_color_as_moved(struct diff_options *o,
 							    &pmb, &pmb_alloc,
 							    &pmb_nr);
 
-			if (contiguous && pmb_nr && last_symbol != l->s)
+			if (contiguous && pmb_nr && last_symbol == l->s)
 				flipped_block = (flipped_block + 1) % 2;
 			else
 				flipped_block = 0;

t/t4015-diff-whitespace.sh

@@ -1442,6 +1442,78 @@ test_expect_success 'detect permutations inside moved code -- dimmed-zebra' '
 	test_cmp expected actual
 '
 
+test_expect_success 'zebra alternate color is only used when necessary' '
+	cat >old.txt <<-\EOF &&
+	line 1A should be marked as oldMoved newMovedAlternate
+	line 1B should be marked as oldMoved newMovedAlternate
+	unchanged
+	line 2A should be marked as oldMoved newMovedAlternate
+	line 2B should be marked as oldMoved newMovedAlternate
+	line 3A should be marked as oldMovedAlternate newMoved
+	line 3B should be marked as oldMovedAlternate newMoved
+	unchanged
+	line 4A should be marked as oldMoved newMovedAlternate
+	line 4B should be marked as oldMoved newMovedAlternate
+	line 5A should be marked as oldMovedAlternate newMoved
+	line 5B should be marked as oldMovedAlternate newMoved
+	line 6A should be marked as oldMoved newMoved
+	line 6B should be marked as oldMoved newMoved
+	EOF
+	cat >new.txt <<-\EOF &&
+	  line 1A should be marked as oldMoved newMovedAlternate
+	  line 1B should be marked as oldMoved newMovedAlternate
+	unchanged
+	  line 3A should be marked as oldMovedAlternate newMoved
+	  line 3B should be marked as oldMovedAlternate newMoved
+	  line 2A should be marked as oldMoved newMovedAlternate
+	  line 2B should be marked as oldMoved newMovedAlternate
+	unchanged
+	  line 6A should be marked as oldMoved newMoved
+	  line 6B should be marked as oldMoved newMoved
+	    line 4A should be marked as oldMoved newMovedAlternate
+	    line 4B should be marked as oldMoved newMovedAlternate
+	  line 5A should be marked as oldMovedAlternate newMoved
+	  line 5B should be marked as oldMovedAlternate newMoved
+	EOF
+	test_expect_code 1 git diff --no-index --color --color-moved=zebra \
+		 --color-moved-ws=allow-indentation-change \
+		 old.txt new.txt >output &&
+	grep -v index output | test_decode_color >actual &&
+	cat >expected <<-\EOF &&
+	<BOLD>diff --git a/old.txt b/new.txt<RESET>
+	<BOLD>--- a/old.txt<RESET>
+	<BOLD>+++ b/new.txt<RESET>
+	<CYAN>@@ -1,14 +1,14 @@<RESET>
+	<BOLD;MAGENTA>-line 1A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;MAGENTA>-line 1B should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 1A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 1B should be marked as oldMoved newMovedAlternate<RESET>
+	 unchanged<RESET>
+	<BOLD;MAGENTA>-line 2A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;MAGENTA>-line 2B should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;BLUE>-line 3A should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;BLUE>-line 3B should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 3A should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 3B should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>  line 2A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>  line 2B should be marked as oldMoved newMovedAlternate<RESET>
+	 unchanged<RESET>
+	<BOLD;MAGENTA>-line 4A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;MAGENTA>-line 4B should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;BLUE>-line 5A should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;BLUE>-line 5B should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;MAGENTA>-line 6A should be marked as oldMoved newMoved<RESET>
+	<BOLD;MAGENTA>-line 6B should be marked as oldMoved newMoved<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 6A should be marked as oldMoved newMoved<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 6B should be marked as oldMoved newMoved<RESET>
+	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>    line 4A should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;YELLOW>+<RESET><BOLD;YELLOW>    line 4B should be marked as oldMoved newMovedAlternate<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 5A should be marked as oldMovedAlternate newMoved<RESET>
+	<BOLD;CYAN>+<RESET><BOLD;CYAN>  line 5B should be marked as oldMovedAlternate newMoved<RESET>
+	EOF
+	test_cmp expected actual
+'
+
 test_expect_success 'cmd option assumes configured colored-moved' '
 	test_config color.diff.oldMoved "magenta" &&
 	test_config color.diff.newMoved "cyan" &&