`try summary` is noisy and slow about try

[mgree]

This should be relatively easy to achieve with fts, which we can use in the following way:

1. fts_open the upperdir
2. Walk through, comparing against the lowerdir. (Cf. overlayfs-tools traverse function.)
   a. On a new directory in the upperdir, use fts_children to compute # of files and their total disk usage.
   b. On a whiteout of what was a directory in the lowerdir, a separate call to fts_open on the lowerdir (or simply calling out to du) will tell else how many files were deleted.
3. Generate the summary and---if requested---a shell script that does the merge. Then the commit routine can just directly call this script.