make update-server-info more robust

Since "git update-server-info" may be called automatically
as part of a push or a "gc --auto", we should be robust
against two processes trying to update it simultaneously.
However, we currently use a fixed tempfile, which means that
two simultaneous writers may step on each other's toes and
end up renaming junk into place.

Let's instead switch to using a unique tempfile via mkstemp.
We do not want to use a lockfile here, because it's OK for
two writers to simultaneously update (one will "win" the
rename race, but that's OK; they should be writing the same
information).

While we're there, let's clean up a few other things:

  1. Detect write errors. Report them and abort the update
     if any are found.

  2. Free path memory rather than leaking it (and clean up
     the tempfile when necessary).

  3. Use the pathdup functions consistently rather than
     static buffers or manually calculated lengths.

This last one fixes a potential overflow of "infofile" in
update_info_packs (e.g., by putting large junk into
$GIT_OBJECT_DIRECTORY). However, this overflow was probably
not an interesting attack vector for two reasons:

  a. The attacker would need to control the environment to
     do this, in which case it was already game-over.

  b. During its setup phase, git checks that the directory
     actually exists, which means it is probably shorter
     than PATH_MAX anyway.

Because both update_info_refs and update_info_packs share
these same failings (and largely duplicate each other), this
patch factors out the improved error-checking version into a
helper function.

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