[gsoc-diff] make trim tail support context

gistfile1.diff

diff --git a/xdiff-interface.c b/xdiff-interface.c
index e1e054e..74a8571 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -108,10 +108,9 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b, long ctx)
 	long trimmed = 0, recovered = 0;
 	char *ap = a->ptr + a->size;
 	char *bp = b->ptr + b->size;
+	char *atop = ap, *prev;
 	long smaller = (a->size < b->size) ? a->size : b->size;
-
-	if (ctx)
-		return;
+	long lines = 0;
 
 	while (blk + trimmed <= smaller && !memcmp(ap - blk, bp - blk, blk)) {
 		trimmed += blk;
@@ -124,6 +123,16 @@ static void trim_common_tail(mmfile_t *a, mmfile_t *b, long ctx)
 			break;
 	a->size -= trimmed - recovered;
 	b->size -= trimmed - recovered;
+
+	ap -= recovered;
+	prev = ap;
+	while (lines < ctx && ap < atop && (ap = memchr(ap, '\n', atop - ap))) {
+		lines++;
+		++ap;
+		a->size += ap - prev;
+		b->size += ap - prev;
+		prev = ap;
+	}
 }
 
 int xdi_diff(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp, xdemitconf_t const *xecfg, xdemitcb_t *xecb)