git-gui: Improve the Windows and Mac OS X shortcut creators

git-gui: Improve the Windows and Mac OS X shortcut creators

We now embed any GIT_* and SSH_* environment variables as well as
the path to the git wrapper executable into the Mac OS X .app file.
This should allow us to restore the environment properly when
we restart.

We also try to use proper Bourne shell single quoting when we can,
as this avoids any sort of problems that might occur due to a path
containing shell metacharacters.

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