git-gui: avoid exception upon Ctrl+T in an empty list

git-gui: avoid exception upon Ctrl+T in an empty list

Previously unstaged files can be staged by clicking on them and then
pressing Ctrl+T. Conveniently, the next unstaged file is selected
automatically so that the unstaged files can be staged by repeatedly
pressing Ctrl+T.

When a user hits Ctrl+T one time too many, though, Git GUI used to throw
this exception:

	expected number but got ""
	expected number but got ""
	    while executing
	"expr {int([lindex [$w tag ranges in_diff] 0])}"
	    (procedure "toggle_or_diff" line 13)
	    invoked from within
	"toggle_or_diff toggle .vpane.files.workdir.list "
	    (command bound to event)

Let's just avoid that by skipping the operation when there are no more
files to stage.

This fixes https://github.com/git-for-windows/git/issues/1060

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>