git-gui: Correctly install to /usr/bin on Cygwin

git-gui: Correctly install to /usr/bin on Cygwin

Mark Levedahl <mlevedahl@gmail.com> noted that installation on Cygwin
to /usr/bin can cause problems with the automatic guessing of our
library location.  The problem is that installation to /usr/bin
means we actually have:

  /usr/bin   = c:\cygwin\bin
  /usr/share = c:\cygwin\usr\share

So git-gui guesses that its library should be found within the
c:\cygwin\share directory, as that is where it should be relative
to the script itself in c:\cygwin\bin.

In my first version of this patch I tried to use `cygpath` to resolve
/usr/bin and /usr/share to test that they were in the same relative
locations, but that didn't work out correctly as we were actually
testing /usr/share against itself, so it always was equal, and we
always used relative paths.  So my original solution was quite wrong.

Mark suggested we just always disable relative behavior on Cygwin,
because of the complexity of the mount mapping problem, so that's
all I'm doing.

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