unmtime: Restore mtimes for olden build systems

Many build systems (e.g: make, cabal) use mtimes to decide whether to rebuild things.

This is very unfortunate in many cases, when mtimes change but content does not.

For example, "git pull --rebase" re-applies all local commits on top of the fresh upstream branch. This touches all local files mtimes even though they mostly end up the same.

unmtime is a utility that records the content hashes and mtimes of all files in .db/files.json (while ignoring files listed in db/blacklist.json). When it detects an mtime changed but the content hash did not, it restores the old mtime.

Running unmtime prior to running "cabal" or "make" can save lots of unneeded rebuilds.

Sending mtime back could confuse build systems that compare mtimes via Ord, and not Eq. So use with care.