reachable from any of our references, keep objects
reachable from listed <head>s.
-EXAMPLE
--------
+EXAMPLES
+--------
To prune objects not used by your repository or another that
borrows from your repository via its
$ git prune $(cd ../another && git rev-parse --all)
------------
-Notes
+NOTES
-----
In most cases, users will not need to call 'git prune' directly, but