bundle: use prefix_filename with bundle path

bundle: use prefix_filename with bundle path

We may take the path to a bundle file as an argument, and
need to adjust the filename based on the prefix we
discovered while setting up the git directory. We do so
manually into a fixed-size buffer, but using
prefix_filename() is the normal way.

Besides being more concise, there are two subtle
improvements:

  1. The original inserted a "/" between the two paths, even
     though the "prefix" argument always has the "/"
     appended. That means that:

       cd subdir && git bundle verify ../foo.bundle

     was looking at (and reporting) subdir//../foo.bundle.
     Harmless, but ugly.  Using prefix_filename() gets this
     right.

  2. The original checked for an absolute path by looking
     for a leading '/'. It should have been using
     is_absolute_path(), which also covers more cases on
     Windows (backslashes and dos drive prefixes).

     But it's easier still to just pass the name to
     prefix_filename(), which handles this case
     automatically.

Note that we'll just leak the resulting buffer in the name
of simplicity, since it needs to last through the duration
of the program anyway.

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