xdiff: ignore empty lines before added functions with -W

If a new function and a preceding empty line is appended, diff -W shows
the previous function in full in order to provide context for that empty
line.  In most languages empty lines between sections are not
interesting in and off themselves and showing a whole extra function for
them is not what we want.

Skip empty lines when checking of the appended chunk starts with a
function line, thereby avoiding to extend the context just for them.

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
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

@@ -117,7 +117,7 @@ test_expect_success ' context includes end' '
 	grep "^[+].*End of first part" appended.diff
 '
 
-test_expect_failure ' context does not include other functions' '
+test_expect_success ' context does not include other functions' '
 	test $(grep -c "^[ +-].*Begin" appended.diff) -le 1
 '
 

xdiff/xemit.c

@@ -155,6 +155,18 @@ static long get_func_line(xdfenv_t *xe, xdemitconf_t const *xecfg,
 	return -1;
 }
 
+static int is_empty_rec(xdfile_t *xdf, long ri)
+{
+	const char *rec;
+	long len = xdl_get_rec(xdf, ri, &rec);
+
+	while (len > 0 && XDL_ISSPACE(*rec)) {
+		rec++;
+		len--;
+	}
+	return !len;
+}
+
 int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 		  xdemitconf_t const *xecfg) {
 	long s1, s2, e1, e2, lctx;
@@ -176,12 +188,18 @@ int xdl_emit_diff(xdfenv_t *xe, xdchange_t *xscr, xdemitcb_t *ecb,
 			/* Appended chunk? */
 			if (i1 >= xe->xdf1.nrec) {
 				char dummy[1];
+				long i2 = xch->i2;
 
 				/*
 				 * We don't need additional context if
-				 * a whole function was added.
+				 * a whole function was added, possibly
+				 * starting with empty lines.
 				 */
-				if (match_func_rec(&xe->xdf2, xecfg, xch->i2,
+				while (i2 < xe->xdf2.nrec &&
+				       is_empty_rec(&xe->xdf2, i2))
+					i2++;
+				if (i2 < xe->xdf2.nrec &&
+				    match_func_rec(&xe->xdf2, xecfg, i2,
 						   dummy, sizeof(dummy)) >= 0)
 					goto post_context_calculation;