Part of the cleanup path for _Init is removing the .repo
directory. However, _Init can fail before creating the .repo directory,
so trying to remove it raises another exception:
fatal: invalid branch name 'refs/changes/53/55053/4'
Traceback (most recent call last):
File "/home/mitchelh/bin/repo", line 775, in <module>
main(sys.argv[1:])
File "/home/mitchelh/bin/repo", line 749, in main
os.rmdir(repodir)
OSError: [Errno 2] No such file or directory: '.repo'
Fix this by only removing .repo if it actually exists.
Change-Id: Ia251d29e9c73e013eb296501d11c36263457e235