git-gui: allow Ctrl+T to toggle multiple paths

git-gui: allow Ctrl+T to toggle multiple paths

It is possible to select multiple files in the "Unstaged Changes" and
the "Staged Changes" lists. But when hitting Ctrl+T, surprisingly only
one entry is handled, not all selected ones.

Let's just use the same code path as for the "Stage To Commit" and the
"Unstage From Commit" menu items.

This fixes https://github.com/git-for-windows/git/issues/1012

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