git-gui: Collapse $env(HOME) to ~/ in recent repositories on Windows

git-gui: Collapse $env(HOME) to ~/ in recent repositories on Windows

Apparently native Tcl/Tk on Windows is using \ as the return value
from [file separator] but [file normalize] on that same system is
using / rather than \ to represent a directory separator.  I really
think that is nuts, but its what is happening.

So we can actually just hardcode our separator to / as all systems
we support (Windows, Mac OS X, UNIX) use /.

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