git-gui: cleanup git-bash menu item

git-gui on Git for Windows creates a menu item to start a git-bash session for the current repository. This menu-item works as desired when git-gui is installed in the Git for Windows (g4w) distribution, but not when run from a different location such as normally done in development. The reason is that git-bash's location is known to be '/git-bash' in the Unix pathname space known to MSYS, but this is not known in the Windows pathname space. Instead, git-gui derives a pathname for git-bash assuming it is at a known relative location. If git-gui is run from a different directory than assumed in g4w, the relative location changes, and git-gui resorts to running a generic bash login session in a Windows console. But, the MSYS system underlying Git for Windows includes the 'cygpath' utility to convert between Unix and Windows pathnames. Let's use this so git-bash's Windows pathname is determined directly from /git-bash. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com> Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Taylor Blau <me@ttaylorr.com>