Bug fix: Git provider on_branch? retains trailing newline

I submitted a patch yesterday for using --abbrev-ref for on_branch?; unfortunately, there is a trailing newline, which causes problems when the repository already exists:

```
Error: /Stage[main]//Vcsrepo[...]: Could not evaluate: Execution of '/usr/local/bin/git rev-parse origin/master
' returned 128: fatal: ambiguous argument 'origin/master
': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
origin/master
```