Fix Depot paths if argument is //foobar style on Windows

Adjust depot path if it is "//depot" converted to "/depot" by Posix path conversion [1]

http://www.mingw.org/wiki/Posix_path_conversion [1]


[How to repeat]
On MinGW environment, run as follows

  $ git p4 clone --detect-branches //depot@all
  Depot paths must start with "//": /depot@all
  Usage: git-p4 clone [options] //depot/path[@revRange]