git-gui: Warn Cygwin users about possible environment issues.

Because the Tcl binary distributed with Cygwin tends to not pass along
its own environment (the env array) to its children, its unlikely that
any Git commands spawned by git-gui will receive the same environment
variables that git-gui itself received from the shell which started it.

If the user is counting on environment variables to pass down, like say
GIT_INDEX_FILE, they may not, so we warn them during git-gui startup
that things may not work out as the user intended.  Perhaps one day
when git-gui and git are running on native Windows (rather than through
the Cygwin emulation layers) things will work better.

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