[BUG] auto-repack exits prematurely, locking other processing out

Hi guys!

It looks like the periodic auto-repack backgrounds itself when it shouldn't
do so.  This causes the command it has triggered as a part of to fail:

==========================================================================
[~/linux](master)$ git pull --rebase
remote: Counting objects: 455, done.
remote: Compressing objects: 100% (64/64), done.
remote: Total 267 (delta 208), reused 262 (delta 203)
Receiving objects: 100% (267/267), 44.43 KiB | 0 bytes/s, done.
Resolving deltas: 100% (208/208), completed with 80 local objects.
From git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux
   4b660a7..f02f79d  master     -> linus/master
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
First, rewinding head to replay your work on top of it...
Applying: perf: tools: fix missing casts for printf arguments.
Applying: vt: emulate 8- and 24-bit colour codes.
fatal: Unable to create '/home/kilobyte/linux/.git/refs/heads/master.lock': File exists.

If no other git process is currently running, this probably means a
git process crashed in this repository earlier. Make sure no other git
process is running and remove the file manually to continue.
Could not move back to refs/heads/master
[~/linux]((no branch, rebasing (null)))$
==========================================================================

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