git-mv: search more precisely for source directory in index

git-mv: search more precisely for source directory in index

A move of a directory should find the entries in the index by
searching for the name _including_ the slash. Otherwise, the
directory can be shadowed by a file when it matches the prefix
and is lexicographically smaller, e.g. "ab.c" shadows "ab/".

Noticed by Sergey Vlasov.

[jc: added Sergey's original reproduction recipe as a test case
 at the end of t7001.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>