diff --follow: do not waste cycles while recursing

diff --follow: do not waste cycles while recursing

The "--follow" logic is called from diff_tree_sha1() function, but the
input trees to diff_tree_sha1() are not necessarily the top-level trees
(compare_tree_entry() calls it while it recursively descends into
subtrees).  When a newly created path lives in somewhere deep in the
source hierarchy, e.g. "platform/", but the rename source is in a totally
different place in the destination hierarchy, e.g. "lang-api/src/com/...",
running "try_to_find_renames()" while base is set to "platform/" is a
wasted call.

We only need to run the rename following at the very top level.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>