register_tempfile(): new function to handle an existing temporary file

register_tempfile(): new function to handle an existing temporary file

Allow an existing file to be registered with the tempfile-handling
infrastructure; in particular, arrange for it to be deleted on program
exit. This can be used if the temporary file has to be created in a
more complicated way than just open(). For example:

* If the file itself needs to be created via the lockfile API
* If it is not a regular file (e.g., a socket)

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>