Add remove partial clone doc

This adds information in the documentation about how to remove the
filterspec from a repository and how to ensure everything is present.

/closes https://gitlab.com/gitlab-org/gitaly/-/issues/2767

doc/topics/git/partial_clone.md

@@ -182,3 +182,47 @@ For more details, see the Git documentation for
    # Checkout master
    git checkout master
    ```
+
+## Remove partial clone filtering
+
+Git repositories with partial clone filtering can have the filtering removed. To
+remove filtering:
+
+1. Fetch everything that has been excluded by the filters, to make sure that the
+   repository is complete. If `git sparse-checkout` was used, use
+   `git sparse-checkout disable` to disable it. See the
+   [`disable` documentation](https://git-scm.com/docs/git-sparse-checkout#Documentation/git-sparse-checkout.txt-emdisableem)
+   for more information.
+
+   Then do a regular `fetch` to ensure that the repository is complete. To check if
+   there are missing objects to fetch, and then fetch them, especially when not using
+   `git sparse-checkout`, the following commands can be used:
+
+   ```shell
+   # Show missing objects
+   git rev-list --objects --all --missing=print | grep -e '^\?'
+
+   # Show missing objects without a '?' character before them (needs GNU grep)
+   git rev-list --objects --all --missing=print | grep -oP '^\?\K\w+'
+
+   # Fetch missing objects
+   git fetch origin $(git rev-list --objects --all --missing=print | grep -oP '^\?\K\w+')
+
+   # Show number of missing objects
+   git rev-list --objects --all --missing=print | grep -e '^\?' | wc -l
+   ```
+
+1. Repack everything. This can be done using `git repack -a -d`, for example. This
+   should leave only three files in `.git/objects/pack/`:
+   - A `pack-<SHA1>.pack` file.
+   - Its corresponding `pack-<SHA1>.idx` file.
+   - A `pack-<SHA1>.promisor` file.
+
+1. Delete the `.promisor` file. The above step should have left only one
+   `pack-<SHA1>.promisor` file, which should be empty and should be deleted.
+
+1. Remove partial clone configuration. The partial clone-related configuration
+   variables should be removed from Git config files. Usually only the following
+   configuration must be removed:
+   - `remote.origin.promisor`.
+   - `remote.origin.partialclonefilter`.