git-gui: Worked around environment variable problems on Windows.

git-gui: Worked around environment variable problems on Windows.

Apparently the Cygwin tclsh/wish executables don't pass the environment
that they inherited onto any children that they invoke.  This causes a
problem for some users during 'git fetch' or 'git push' as critical
environment variables like GIT_SSH and SSH_AUTH_SOCK aren't available
to the git processes.

So we work around this by forcing sh to start a login shell, thus
reloading the user's environment, then cd to the current directory,
and finally start the requested process.  Of course this won't
correctly handle any transient environment variables that were
inherited but were not supplied by the user's login shell.

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