findleaves.py: prevent recursion into symlink loops

Keep a set of all visited inodes, and prevent recursing into a symlink
to an already visited inode.

Test: m -j
Test: compare `build/tools/findleaves.py --prune=.repo --prune=.git --mindepth=2 --dir=. Android.mk` before and after
Change-Id: Ied14c40d3066ef9f8e8a2b1535f56f7bbbbd0ab6