[PATCH 0/4] improve symbolic-ref robustness

I noticed that an interrupt "git symbolic-ref" will not clean up "HEAD.lock". So I started this series as an attempt to convert create_symref() to "struct lock_file" to get the usual tempfile cleanup.

But I found a few other points of interest. The biggest is that git-symbolic-ref does not actually propagate errors in its exit code. Whoops. That's fixed in the first patch, which could go separately to "maint".

The rest of it is fairly dependent on master because of the refs/files-backend.c code movement. I can backport it if we really want it for maint.

  [1/4]: symbolic-ref: propagate error code from create_symref()
  [2/4]: t1401: test reflog creation for git-symbolic-ref
  [3/4]: create_symref: modernize variable names
  [4/4]: create_symref: use existing ref-lock code

-Peff