git-gui: Avoid Tcl error in popup menu on diff viewer

git-gui: Avoid Tcl error in popup menu on diff viewer

If there is no path currently shown in the diff viewer then we
were getting Tcl errors anytime the user right-clicked on the
diff viewer to bring up its popup menu.  The bug here is caused
by trying to get the file_state for the empty string; this path
is never seen so we never have file_state for it.  In such cases
we now disable the Stage Hunk For Commit option.

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