git-mv needs to be run from the base directory so that
the check if a file is under revision also covers files
outside of a subdirectory. Previously, e.g. in the git repo,

  cd Documentation; git-mv ../README .

produced the error

  Error: '../README' not under version control

The test is extended for this case; it previously only tested
one direction.
Signed-off-by: NJosef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: NJunio C Hamano <junkio@cox.net>