git mv: try harder to keep index entries intact

git mv: try harder to keep index entries intact

Some filesystems change the ctime during a rename(), for technical
reasons.  Since this is the only change, the contents need not be
rehashed.  So just update the ctime after renaming the entry.

This change requires rename_index_entry_at() to return the new position.

To test this properly, you need to run t7001 with the environment variable
TEST_CTIME_WITH_SLEEP set non-empty, since there is no way to manipulate
the ctime directly; we have to sleep.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>