Fix hang following failed finish-commit

[jdoliner]

Steps to reproduce:

$ pachctl create-repo foo
$ pachctl finish-commit foo bar # commit doesn't exist
$ pachctl start-commit foo # now hangs

looks like we have a locking issue when calls to finish-commit fail.

[mattnenterprise]

This currently works for me:

$ pachctl create-repo foo
$ pachctl finish-commit foo bar
rpc error: code = 2 desc = "commit foo/bar not found"
$ pachctl start-commit foo
af44ba6e7aba4e399a563ca5397f3586

[jdoliner]

Nice, makes sense that this would be fixed too given the new storage backend. Closing.