git-gui: Include version check and test for tearoff menu entry

The --all option for git fetch was added in v1.6.6 so ensure we have a usable version before adding
the menu items.
Sometimes people use tearoff menus and these offset the entry indices by one.

Acked-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Merge branch 'hv/fixup-fetch-from-all' into HEAD

Conflicts:
	git-gui/lib/remote.tcl