range-diff: internally force `diff.noprefix=true`

When parsing the diffs, `range-diff` expects to see the prefixes `a/`
and `b/` in the diff headers.

These prefixes can be forced off via the config setting
`diff.noprefix=true`. As `range-diff` is not prepared for that
situation, this will cause a segmentation fault.

Let's avoid that by passing the `--no-prefix` option to the `git log`
process that generates the diffs that `range-diff` wants to parse.
And of course expect the output to have no prefixes, then.

Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -52,6 +52,7 @@ static int read_patches(const char *range, struct string_list *list)
 
 	argv_array_pushl(&cp.args, "log", "--no-color", "-p", "--no-merges",
 			"--reverse", "--date-order", "--decorate=no",
+			"--no-prefix",
 			/*
 			 * Choose indicators that are not used anywhere
 			 * else in diffs, but still look reasonable
@@ -111,7 +112,7 @@ static int read_patches(const char *range, struct string_list *list)
 			if (!util->diff_offset)
 				util->diff_offset = buf.len;
 			line[len - 1] = '\n';
-			len = parse_git_diff_header(&root, &linenr, 1, line,
+			len = parse_git_diff_header(&root, &linenr, 0, line,
 						    len, size, &patch);
 			if (len < 0)
 				die(_("could not parse git header '%.*s'"), (int)len, line);

t/t3206-range-diff.sh

@@ -354,4 +354,8 @@ test_expect_success 'format-patch --range-diff as commentary' '
 	grep "> 1: .* new message" 0001-*
 '
 
+test_expect_success 'range-diff overrides diff.noprefix internally' '
+	git -c diff.noprefix=true range-diff HEAD^...
+'
+
 test_done