xdl_merge(): fix a segmentation fault when refining conflicts

The function xdl_refine_conflicts() tries to break down huge
conflicts by doing a diff on the conflicting regions. However,
this does not make sense when one side is empty.

Worse, when one side is not only empty, but after EOF, the code
accessed unmapped memory.

Noticed by Luben Tuikov, Shawn Pearce and Alexandre Julliard, the
latter providing a test case.
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t6023-merge-file.sh

@@ -112,5 +112,27 @@ EOF
 test_expect_success "expected conflict markers, with -L" \
 	"diff -u test.txt expect.txt"
 
+sed "s/ tu / TU /" < new1.txt > new5.txt
+test_expect_failure "conflict in removed tail" \
+	"git-merge-file -p orig.txt new1.txt new5.txt > out"
+
+cat > expect << EOF
+Dominus regit me,
+et nihil mihi deerit.
+In loco pascuae ibi me collocavit,
+super aquam refectionis educavit me;
+animam meam convertit,
+deduxit me super semitas jusitiae,
+propter nomen suum.
+<<<<<<< orig.txt
+=======
+Nam et si ambulavero in medio umbrae mortis,
+non timebo mala, quoniam TU mecum es:
+virga tua et baculus tuus ipsa me consolata sunt.
+>>>>>>> new5.txt
+EOF
+
+test_expect_success "expected conflict markers" "diff -u expect out"
+
 test_done
 

xdiff/xmerge.c

@@ -190,6 +190,10 @@ static int xdl_refine_conflicts(xdfenv_t *xe1, xdfenv_t *xe2, xdmerge_t *m,
 		if (m->mode)
 			continue;
 
+		/* no sense refining a conflict when one side is empty */
+		if (m->chg1 == 0 || m->chg2 == 0)
+			continue;
+
 		/*
 		 * This probably does not work outside git, since
 		 * we have a very simple mmfile structure.