diffcore-rename: limit dir_rename_counts computation to relevant dirs

diffcore-rename: limit dir_rename_counts computation to relevant dirs

We are using dir_rename_counts to count the number of other directories
that files within a directory moved to.  We only need this information
for directories that disappeared, though, so we can return early from
update_dir_rename_counts() for other paths.

If dirs_removed is passed to diffcore_rename_extended(), then it
provides the relevant bits of information for us to limit this counting
to relevant dirs.  If dirs_removed is not passed, we would need to
compute some replacement in order to do this limiting.  Introduce a new
info->relevant_source_dirs variable for this purpose, even though at
this stage we will only set it to dirs_removed for simplicity.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>