git-mv: improve error message for conflicted file

git-mv: improve error message for conflicted file

'git mv' has always complained about renaming a conflicted
file, as it cannot handle multiple index entries for one file.
However, the error message it uses has been the same as the
one for an untracked file:

    fatal: not under version control, src=...

which is patently wrong.  Distinguish the two cases and
add a test to make sure we produce the correct message.

Signed-off-by: Chris Torek <chris.torek@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>