Merge branch 'pw/p4-branch-fixes'

Fix "git p4" around branch handling.

* pw/p4-branch-fixes:
  git p4: fix submit when no master branch
  git p4 test: keep P4CLIENT changes inside subshells
  git p4: fix sync --branch when no master branch
  git p4: fail gracefully on sync with no master branch
  git p4: rearrange self.initialParent use
  git p4: allow short ref names to --branch
  git p4 doc: fix branch detection example
  git p4: clone --branch should checkout master
  git p4: verify expected refs in clone --bare test
  git p4: create p4/HEAD on initial clone
  git p4: inline listExistingP4GitBranches
  git p4: add comments to p4BranchesInGit
  git p4: rearrange and simplify hasOrigin handling
  git p4: test sync/clone --branch behavior

diff --cc git-p4.py
Simple merge