xdiff: don't trim common tail with -W

The function trim_common_tail() exits early if context lines are
requested.  If -U0 and -W are specified together then it can still trim
context lines that might belong to a changed function.  As a result
that function is shown incompletely.

Fix that by calling trim_common_tail() only if no function context or
fixed context is requested.  The parameter ctx is no longer needed now;
remove it.

While at it fix an outdated comment as well.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4051-diff-function-context.sh

@@ -145,7 +145,7 @@ test_expect_success ' context includes begin' '
 	grep "^ .*Begin of first part" long_common_tail.diff
 '
 
-test_expect_failure ' context includes end' '
+test_expect_success ' context includes end' '
 	grep "^ .*End of second part" long_common_tail.diff
 '
 

xdiff-interface.c

@@ -100,9 +100,9 @@ static int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf)
 
 /*
  * Trim down common substring at the end of the buffers,
- * but leave at least ctx lines at the end.
+ * but end on a complete line.
  */
-static void trim_common_tail(mmfile_t *a, mmfile_t *b, long ctx)
+static void trim_common_tail(mmfile_t *a, mmfile_t *b)
 {
 	const int blk = 1024;
 	long trimmed = 0, recovered = 0;
@@ -110,9 +110,6 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b, long ctx)
 	char *bp = b->ptr + b->size;
 	long smaller = (a->size < b->size) ? a->size : b->size;
 
-	if (ctx)
-		return;
-
 	while (blk + trimmed <= smaller && !memcmp(ap - blk, bp - blk, blk)) {
 		trimmed += blk;
 		ap -= blk;
@@ -134,7 +131,8 @@ int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t co
 	if (mf1->size > MAX_XDIFF_SIZE || mf2->size > MAX_XDIFF_SIZE)
 		return -1;
 
-	trim_common_tail(&a, &b, xecfg->ctxlen);
+	if (!xecfg->ctxlen && !(xecfg->flags & XDL_EMIT_FUNCCONTEXT))
+		trim_common_tail(&a, &b);
 
 	return xdl_diff(&a, &b, xpp, xecfg, xecb);
 }