merge-recursive: clean up get_renamed_dir_portion()
Dscho noted a few things making this function hard to follow.
Restructure it a bit and add comments to make it easier to follow. The
restructurings include:
* There was a special case if-check at the end of the function
checking whether someone just renamed a file within its original
directory, meaning that there could be no directory rename involved.
That check was slightly convoluted; it could be done in a more
straightforward fashion earlier in the function, and can be done
more cheaply too (no call to strncmp).
* The conditions for advancing end_of_old and end_of_new before
calling strchr were both confusing and unnecessary. If either
points at a '/', then they need to be advanced in order to find the
next '/'. If either doesn't point at a '/', then advancing them one
char before calling strchr() doesn't hurt. So, just rip out the
if conditions and advance both before calling strchr().
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>