docs: explain that we don't support shallow clones (#675)

docs/git-compatibility.md

@@ -56,7 +56,9 @@ a comparison with Git, including how workflows are different, see the
 * **Submodules: No.** They will not show up in the working copy, but they will
   not be lost either.
 * **Partial clones: No.** We use the [libgit2](https://libgit2.org/) library,
-  which doesn't have support for partial clones.
+  which [doesn't have support for partial clones](https://github.com/libgit2/libgit2/issues/5564).
+* **Shallow clones: No.** We use the [libgit2](https://libgit2.org/) library,
+  which [doesn't have support for shallow clones](https://github.com/libgit2/libgit2/issues/3058).
 * **git-worktree: No.** However, there's native support for multiple working
   copies backed by a single repo. See the `jj workspace` family of commands.
 * **Sparse checkouts: No.** However, there's native support for sparse