env: implement the repodiff command

 sample usage:
   repodiff ics
   repodiff github/ics --numstat
   repodiff android-4.0.4_r1.2 android-4.0.4_r2

Change-Id: Id1c1e504c75676707e59f051fd5ff9ce748d4297
Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>

build: Add repodiff to list of commands