Refactor index-pack "keep $sha1" handling for reuse

Refactor index-pack "keep $sha1" handling for reuse

There is a subtle (but important) linkage between receive-pack and
index-pack that allows index-pack to create a packfile but protect
it from being deleted by a concurrent `git repack -a -d` operation.
The linkage works by having index-pack mark the newly created pack
with a ".keep" file and then it passes the SHA-1 name of that new
packfile to receive-pack along its stdout channel.

The receive-pack process must unkeep the packfile by deleting the
.keep file, but can it can only do so after all elgible refs have
been updated in the receiving repository.  This ensures that the
packfile is either kept or its objects are reachable, preventing
a concurrent repacker from deleting the packfile before it can
determine that its objects are actually needed by the repository.

The new builtin-fetch code needs to perform the same actions if
it choose to run index-pack rather than unpack-objects, so I am
moving this code out to its own function where both receive-pack
and fetch-pack are able to invoke it when necessary.  The caller
is responsible for deleting the returned ".keep" and freeing the
path if the returned path is not NULL.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>