git-mv: fix moves into a subdir from outside

git-mv: fix moves into a subdir from outside

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: Josef Weidendorfer <redacted>
Signed-off-by: Junio C Hamano <redacted>