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.

Note that this affects only the beginning of the file: when a hunk is
plit into two, the first can change the context so that the second hunk
thinks it should match the beginning of the file, but it no longer does
because the first hunk was already applied. The same is not true for the
end of the file, as the hunks are applied in order (by line numbers):
the hunk that changes the end of the file is by definition the last hunk
to be applied.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

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