Merge branch 'ld/p4-detached-head' into maint

Make git-p4 work on a detached head.

* ld/p4-detached-head:
git-p4: work with a detached head
git-p4: add option to system() to return subshell status
git-p4: add failing test for submit from detached head

diff --cc git-p4.py
Simple merge