Merge branch 'ml/cygwin-fixes'

Remove some code supporting ancient Cygwin Tcl/Tk versions. Also fix
exploring working directory and making desktop shortcuts on Cygwin.

* ml/cygwin-fixes:
  git-gui - use mkshortcut on Cygwin
  git-gui - use cygstart to browse on Cygwin
  git-gui - remove obsolete Cygwin specific code
  git gui Makefile - remove Cygwin modifications