User level Git errors shouldn't produce stack traces about git-repo HOT 2 OPEN

You also get a stack trace when running `repo init` without your git identity 
set. To repro:

1. Start from base install of Ubuntu 14.10
2. `sudo apt-get install git python`
3. `wget https://storage.googleapis.com/git-repo-downloads/repo`
4. `chmod +x repo`
5. `mkdir foo`
6. `cd foo`
7. `repo init -u ssh://...`
8. boom

Traceback (most recent call last):
  File "/home/test/sel4test/.repo/repo/main.py", line 500, in <module>
    _Main(sys.argv[1:])
  File "/home/test/sel4test/.repo/repo/main.py", line 476, in _Main
    result = repo._Run(argv) or 0
  File "/home/test/sel4test/.repo/repo/main.py", line 155, in _Run
    result = cmd.Execute(copts, cargs)
  File "/home/test/sel4test/.repo/repo/subcmds/init.py", line 395, in Execute
    self._ConfigureUser()
  File "/home/test/sel4test/.repo/repo/subcmds/init.py", line 289, in _ConfigureUser
    name  = self._Prompt('Your Name', mp.UserName)
  File "/home/test/sel4test/.repo/repo/project.py", line 650, in UserName
    self._LoadUserIdentity()
  File "/home/test/sel4test/.repo/repo/project.py", line 663, in _LoadUserIdentity
    u = self.bare_git.var('GIT_COMMITTER_IDENT')
  File "/home/test/sel4test/.repo/repo/project.py", line 2483, in runner
    p.stderr))
error.GitError: manifests var:
*** Please tell me who you are.

Run

  git config --global user.email "[email protected]"
  git config --global user.name "Your Name"

to set your account's default identity.