refdb: don't report failure for expected errors
refdb: don't report failure for expected errors

There might be a few threads or processes working with references
concurrently, so fortify the code to ignore errors which come from
concurrent access which do not stop us from continuing the work.

This includes ignoring an unlinking error. Either someone else removed
it or we leave the file around. In the former case the job is done, and
in the latter case, the ref is still in a valid state.

stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2