git-gui: Support native Win32 Tcl/Tk under Cygwin

git-gui: Support native Win32 Tcl/Tk under Cygwin

Cygwin has been stuck on the 8.4.1 version of Tcl/Tk for quite some
time, even though the main Tcl/Tk distribution is already shipping
an 8.4.15.  The problem is Tcl/Tk no longer supports Cygwin so
apparently building the package for Cygwin is now a non-trivial task.

Its actually quite easy to build the native Win32 version of Tcl/Tk
by compiling with the -mno-cygwin flag passed to GCC but this means
we lose all of the "fancy" Cygwin path translations that the Tcl
library was doing for us.  This is particularly an issue when we
are trying to start git-gui through the git wrapper as the git
wrapper is passing off a Cygwin path for $0 and Tcl cannot find
the startup script or the library directory.

We now use `cygpath -m -a` to convert the UNIX style paths to Windows
style paths in our startup script if we are building on Cygwin.
Doing so allows either the Cygwin-ized Tcl/Tk 8.4.1 that comes with
Cygwin or a manually built 8.4.15 that is running the pure Win32
implementation to read our script.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>