Merge branch 'py/git-gui-updates'

Merge branch 'py/git-gui-updates'

Git GUI updates.

* py/git-gui-updates:
  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
  Makefiles: change search through $(MAKEFLAGS) for GNU make 4.4
  Work around Tcl's default `PATH` lookup
  Move the `_which` function (almost) to the top
  Move is_<platform> functions to the beginning
  is_Cygwin: avoid `exec`ing anything
  windows: ignore empty `PATH` elements
  git-gui: Fix a typo in README