git-gui: Avoid hardcoded Windows paths in Cygwin package files

git-gui: Avoid hardcoded Windows paths in Cygwin package files

When we are being built by the Cygwin package maintainers we need to
embed the POSIX path to our library files and not the Windows path.
Embedding the Windows path means all end-users who install our Cygwin
package would be required to install Cygwin at the same Windows path
as the package maintainer had Cygwin installed to.  This requirement
is simply not user-friendly and may be infeasible for a large number
of our users.

We now try to auto-detect if the Tcl/Tk binary we will use at runtime
is capable of translating POSIX paths into Windows paths the same way
that cygpath does the translations.  If the Tcl/Tk binary gives us the
same results then it understands the Cygwin path translation process
and should be able to read our library files from a POSIX path name.

If it does not give us the same answer as cygpath then the Tcl/Tk
binary might actually be a native Win32 build (one that is not
linked against Cygwin) and thus requires the native Windows path
to our library files.  We can assume this is not a Cygwin package
as the Cygwin maintainers do not currently ship a pure Win32 build
of Tcl/Tk.

Reported on the git mailing list by Jurko Gospodnetić.

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