apply: handle filenames with double slashes better

apply: handle filenames with double slashes better

When there are duplicated slashes in pathnames, like this:

	--- a/perl//Git.pm
	+++ b/perl//Git.pm
	@@ -1358,3 +1358,4 @@

	 1; # Famous last words
	+# test

the paths gleaned from the patch header won't be found in the index and
cause "apply --index" and "apply --cached" to fail.

Fix this by squashing the duplicated slashes upon input.

Signed-off-by: Michal Marek <redacted>
Signed-off-by: Junio C Hamano <redacted>