git-gui: Correctly handle GIT_DIR environment variable.

git-gui: Correctly handle GIT_DIR environment variable.

Some users may want to start us by running "git --git-dir=... gui"
rather than trying to cd into the directory first.  This is especially
true if they want to just make a shortcut to our executable on Windows
and always have that associated with a certain repository.

Since Tcl on Windows throws away our environment and doesn't pass it
down to the child process correctly we cannot call git-rev-parse to
get the GIT_DIR environment variable.  So instead we ask for it
specifically ourselves; if its not defined then we ask rev-parse.
This should actually reduce startup by 1 fork/exec if we were started
as "git gui" as GIT_DIR will be set for us.

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