git-gui: Trim trailing slashes from untracked submodule names

git-gui: Trim trailing slashes from untracked submodule names

Oddly enough `git ls-files --others` supplies us the name of an
untracked submodule by including the trailing slash but that
same git version will not accept the name with a trailing slash
through `git update-index --stdin`.  Stripping off that final
slash character before loading it into our file lists allows
git-gui to stage changes to submodules just like any other file.

This change should give git-gui users some basic submodule support,
but it is strictly at the plumbing level as we do not actually know
about calling the git-submodule porcelain that is a recent addition
to git 1.5.3.

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