git-gui - remove obsolete Cygwin specific code

git-gui - remove obsolete Cygwin specific code

In the current git release, git-gui runs on Cygwin without enabling any
of git-gui's Cygwin specific code.  This happens as the Cygwin specific
code in git-gui was (mostly) written in 2007-2008 to work with Cygwin's
then supplied Tcl/Tk which was an incompletely ported variant of the
8.4.1 Windows Tcl/Tk code.  In March, 2012, that 8.4.1 package was
replaced with a full port based upon the upstream unix/X11 code,
since maintained up to date. The two Tcl/Tk packages are completely
incompatible, and have different signatures.

When Cygwin's Tcl/Tk signature changed in 2012, git-gui no longer
detected Cygwin, so did not enable Cygwin specific code, and the POSIX
environment provided by Cygwin since 2012 supported git-gui as a generic
unix. Thus, no-one apparently noticed the existence of incompatible
Cygwin specific code.

However, since commit c5766eae6f in the git-gui source tree
(https://github.com/prati0100/git-gui, master at a5005ded), and not yet
pulled into the git repository, the is_Cygwin function does detect
Cygwin using the unix/X11 Tcl/Tk.  The Cygwin specific code is enabled,
causing use of Windows rather than unix pathnames, and enabling
incorrect warnings about environment variables that were relevant only
to the old Tcl/Tk.  The end result is that (upstream) git-gui is now
incompatible with Cygwin.

So, delete Cygwin specific code (code protected by "if is_Cygwin") that
is not needed in any form to work with the unix/X11 Tcl/Tk.

Cygwin specific code required to enable file browsing and shortcut
creation is not addressed in this patch, does not currently work, and
invocation of those items may leave git-gui in a confused state.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>