git p4: bug - branch detection broken on empty branches

[Dan Porter <dpreid@gmail.com>]

As part of my work to help get git-p4 close to bug-free before Git
2.0, I'm posting all bugs and patches to this mailing list. Please
direct me elsewhere if this is incorrect.

When trying to clone a particular directory from a depot, that
contains one or more branches that contain no commits for that
directory, branch detection is broken and results in a failed clone.

fatal: ambiguous argument
'refs/remotes/p4/silly_project_branch/trunk': unknown revision or path
not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Traceback (most recent call last):
  File "/home/dreid/bin/git-p4", line 3295, in ?
    main()
  File "/home/dreid/bin/git-p4", line 3289, in main
    if not cmd.run(args):
  File "/home/dreid/bin/git-p4", line 3163, in run
    if not P4Sync.run(self, depotPaths):
  File "/home/dreid/bin/git-p4", line 3016, in run
    self.importChanges(changes)
  File "/home/dreid/bin/git-p4", line 2678, in importChanges
    blob = self.searchParent(parent, branch, tempBranch)
  File "/home/dreid/bin/git-p4", line 2600, in searchParent
    for blob in read_pipe_lines(["git", "rev-list", "--reverse",
  File "/home/dreid/bin/git-p4", line 155, in read_pipe_lines
    die('Command failed: %s' % str(c))
  File "/home/dreid/bin/git-p4", line 106, in die
    raise Exception(msg)
Exception: Command failed: ['git', 'rev-list', '--reverse',
'--no-merges', 'refs/remotes/p4/silly_project_branch/trunk']

Original command:
$ git-p4 clone //insane_depot/projects/Exchange/CompanyName/silly_project_branch@all
silly-project --detect-branches -v