t9300: document fast-import empty path issues

t9300: document fast-import empty path issues

When given an empty path, fast-import sometimes reports "missing"
instead of using the root tree object.  On top of this, for "ls" and
file copy (but not move) it dies with "Empty path component found in
input".

Document this behaviour with failing test cases.

Reported-by: Dave Abrahams <dave@boostpro.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>