Fixes issue #1056.
When you try to push `master` but also have a file named `master`, we run
`git rev-list ... master`. This is ambiguous, because `rev-list` accepts
paths or refs and `git` is unsure whether you mean the ref `master` or the
path `master.
Instead, run `git rev-list ... master --`, which is unambiguous.
This adds a failing test (trying to push a file named `master` to LFS), then
fixes the test by appending `--` to the command.
