Git.pm: Add faculties to allow temp files to be cached

Git.pm: Add faculties to allow temp files to be cached

This patch offers a generic interface to allow temp files to be
cached while using an instance of the 'Git' package. If many
temp files are created and destroyed during the execution of a
program, this caching mechanism can help reduce the amount of
files created and destroyed by the filesystem.

The temp_acquire method provides a weak guarantee that a temp
file will not be stolen by subsequent requests. If a file is
locked when another acquire request is made, a simple error is
thrown.

Signed-off-by: Marcus Griep <marcus@griep.us>
Acked-by: Eric Wong <normalperson@yhbt.net>