Make the pack-refs interfaces usable from outside

Make the pack-refs interfaces usable from outside

This just basically creates a "pack_refs()" function that could be used by
anybody. You pass it in the flags you want as a bitmask (PACK_REFS_ALL and
PACK_REFS_PRUNE), and it will do all the heavy lifting.

Of course, it's still static, and it's all in the builtin-pack-refs.c
file, so it's not actually visible to the outside, but the next step would
be to just move it all to a library file (probably refs.c) and expose it.

Then we could easily make "git gc" do this too.

While I did it, I also made it check the return value of the fflush and
fsync stage, to make sure that we don't overwrite the old packed-refs file
with something that got truncated due to write errors!

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>