diff -c/--cc: do not include uninteresting deletion before leading context

When we include a few uninteresting lines before the interesting ones as
context, we are only interested in seeing the surviving lines themselves
and not the deleted lines that are before them.  Mark the added leading
context lines in give_context() and not show deleted lines form them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -84,6 +84,7 @@ struct sline {
 	/* bit 0 up to (N-1) are on if the parent has this line (i.e.
 	 * we did not change it).
 	 * bit N is used for "interesting" lines, including context.
+	 * bit (N+1) is used for "do not show deletion before this".
 	 */
 	unsigned long flag;
 	unsigned long *p_lno;
@@ -308,6 +309,7 @@ static int give_context(struct sline *sline, unsigned long cnt, int num_parent)
 {
 	unsigned long all_mask = (1UL<<num_parent) - 1;
 	unsigned long mark = (1UL<<num_parent);
+	unsigned long no_pre_delete = (2UL<<num_parent);
 	unsigned long i;
 
 	/* Two groups of interesting lines may have a short gap of
@@ -329,7 +331,7 @@ static int give_context(struct sline *sline, unsigned long cnt, int num_parent)
 
 		/* Paint a few lines before the first interesting line. */
 		while (j < i)
-			sline[j++].flag |= mark;
+			sline[j++].flag |= mark | no_pre_delete;
 
 	again:
 		/* we know up to i is to be included.  where does the
@@ -502,6 +504,7 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 		       int use_color)
 {
 	unsigned long mark = (1UL<<num_parent);
+	unsigned long no_pre_delete = (2UL<<num_parent);
 	int i;
 	unsigned long lno = 0;
 	const char *c_frag = diff_get_color(use_color, DIFF_FRAGINFO);
@@ -581,7 +584,7 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 			int j;
 			unsigned long p_mask;
 			sl = &sline[lno++];
-			ll = sl->lost_head;
+			ll = (sl->flag & no_pre_delete) ? NULL : sl->lost_head;
 			while (ll) {
 				fputs(c_old, stdout);
 				for (j = 0; j < num_parent; j++) {