git-prune - Prune all unreachable objects from the object database
git prune [-n] [-v] [--progress] [--expire <time>] [--] [<head>...]
In most cases, users should run git gc, which calls git prune. See the section "NOTES", below.
This runs git fsck --unreachable using all the refs available in refs/, optionally with additional set of objects specified on the command line, and prunes all unpacked objects unreachable from any of these head objects from the object database. In addition, it prunes the unpacked objects that are also found in packs by running git prune-packed. It also removes entries from .git/shallow that are not reachable by any ref.
Note that unreachable, packed objects will remain. If this is not desired, see git-repack(1).
To prune objects not used by your repository or another that borrows from your repository via its .git/objects/info/alternates:
$ git prune $(cd ../another && git rev-parse --all)
In most cases, users will not need to call git prune directly, but should instead call git gc, which handles pruning along with many other housekeeping tasks.
For a description of which objects are considered for pruning, see git fsck's --unreachable option.
Part of the git(1) suite