line-log: fix crash when --first-parent is used

line-log tries to access all parents of a commit, but only the first
parent has been loaded if "--first-parent" is specified, resulting
in a crash.

Limit the number of parents to one if "--first-parent" is specified.

Reported-by: Eric N. Vander Weele <ericvw@gmail.com>
Signed-off-by: Tzvetan Mikov <tmikov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-log.c

@@ -1164,6 +1164,9 @@ static int process_ranges_merge_commit(struct rev_info *rev, struct commit *comm
 	int i;
 	int nparents = count_parents(commit);
 
+	if (nparents > 1 && rev->first_parent_only)
+		nparents = 1;
+
 	diffqueues = xmalloc(nparents * sizeof(*diffqueues));
 	cand = xmalloc(nparents * sizeof(*cand));
 	parents = xmalloc(nparents * sizeof(*parents));

t/t4211-line-log.sh

@@ -77,4 +77,9 @@ test_expect_success '-L {empty-range} (first -L)' '
 	git log -L$n:b.c
 '
 
+test_expect_success '-L with --first-parent and a merge' '
+	git checkout parallel-change &&
+	git log --first-parent -L 1,1:b.c
+'
+
 test_done