git-gui: Abstract out windows platform test to is_Windows proc.
git-gui: Abstract out windows platform test to is_Windows proc.

Like the is_MacOSX proc we shouldn't keep repeating the platform test
for Windows.  Instead abstract the code out into a procedure and use
the procedure whenever we need to do something special.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>
destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git