Add other git-tools (optional)

implements other git-core/git-<tools> in Android.mk

build them with 'mma' in external/git folder,
or 'mmp -B' to push them directly on a CyanogenMod Repo...

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>
xdiff: fix noisy sign warnings

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>