Do not remove anything; just report what it would
+ Do not interpret any more arguments as options.
+ Instead of keeping objects
+ reachable from any of our references, keep objects
+ reachable from only listed <head>s.
+Note that the explicitly named <head>s are *not* appended to the
+default set of references, but they replace them. In general you
+would want to say `git prune $(git-rev-parse --all) extra1
+extra2` to keep chains of commits leading to extra1, extra2,
+... in addition to what are reachable from your own refs.
+Saying `git prune extra1 extra2` would *lose* objects reachable
+only from the usual refs, which is usually not what you want.
+To prune objects not used by your repository and another that
+borrows from your repository via its
+$ git prune $(git-rev-parse --all) \
+ $(cd ../another && $(git-rev-parse --all))