Merge branch 'mm/p4-unshelve-windows-fix' into maint

The command line to invoke a "git cat-file" command from inside
"git p4" was not properly quoted to protect a caret and running a
broken command on Windows, which has been corrected.

* mm/p4-unshelve-windows-fix:
  p4 unshelve: fix "Not a valid object name HEAD0" on Windows