apply --allow-overlap: fix a corner case

Yes, yes, this is supposed to be only a band-aid option for `git add -p`
not Doing The Right Thing. But as long as we carry the `--allow-overlap`
option, we might just as well get it right.

This fixes the case where one hunk inserts a line before the first line,
and is followed by a hunk whose context overlaps with the first one's
and which appends a line at the end.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.c

@@ -2661,6 +2661,16 @@ static int find_pos(struct apply_state *state,
 	unsigned long backwards, forwards, current;
 	int backwards_lno, forwards_lno, current_lno;
 
+	/*
+	 * When running with --allow-overlap, it is possible that a hunk is
+	 * seen that pretends to start at the beginning (but no longer does),
+	 * and that *still* needs to match the end. So trust `match_end` more
+	 * than `match_beginning`.
+	 */
+	if (state->allow_overlap && match_beginning && match_end &&
+	    img->nr - preimage->nr != 0)
+		match_beginning = 0;
+
 	/*
 	 * If match_beginning or match_end is specified, there is no
 	 * point starting from a wrong line that will never match and