diff: strip extra "/" when stripping prefix

diff: strip extra "/" when stripping prefix

There are two ways a user might want to use "diff --relative":

  1. For a file in a directory, like "subdir/file", the user
     can use "--relative=subdir/" to strip the directory.

  2. To strip part of a filename, like "foo-10", they can
     use "--relative=foo-".

We currently handle both of those situations. However, if the user passes
"--relative=subdir" (without the trailing slash), we produce inconsistent
results. For the unified diff format, we collapse the double-slash of
"a//file" correctly into "a/file". But for other formats (raw, stat,
name-status), we end up with "/file".

We can do what the user means here and strip the extra "/" (and only a
slash).  We are not hurting any existing users of (2) above with this
behavior change because the existing output for this case was nonsensical.

Patch by Jakub, tests and commit message by Jeff King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>