apply: handle Subversion diffs with /dev/null gracefully

Subversion generates diffs that can contain lines like this one:

	--- /dev/null  (nonexistent)

Let's teach Git's apply machinery to handle such a line gracefully.

This fixes https://github.com/git-for-windows/git/isues/1489

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

apply.c

@@ -975,7 +975,7 @@ static int gitdiff_verify_name(struct apply_state *state,
 		}
 		free(another);
 	} else {
-		if (!starts_with(line, "/dev/null\n"))
+		if (!is_dev_null(line))
 			return error(_("git apply: bad git-diff - expected /dev/null on line %d"), state->linenr);
 	}