git_backend: replace git2::Repository with gix::Repository

My gut feeling is that gitoxide aims to be more transparent than libgit2. We'll
need to know more about the underlying Git data model.

Random comments on gix API:

 * gix::Repository provides API similar to git2::Repository, but has less
   "convenient" functions. For example, we need to use .find_object() +
   .try_to/into_<kind>() instead of .find_<kind>().
 * gix::Object, Blob, etc. own raw data as bytes. gix::object and gix::objs
   types provide high-level views on such data.
 * Tree building is pretty low-level compared to git2.
 * gix leverages bstr (i.e. bytes) extensively.

It's probably not difficult to migrate git::import/export_refs(). It might
help eliminate the startup overhead of libssl initialization. The gix-based
GitBackend appears to be a bit faster, but that wouldn't practically matter.

martinvonz#2316

cli/tests/test_edit_command.rs

@@ -104,7 +104,7 @@ fn test_edit_current_wc_commit_missing() {
         .assert()
         .code(255);
     insta::assert_snapshot!(get_stderr_string(&assert), @r###"
-    Internal error: Failed to edit a commit: Current working-copy commit not found: Object 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 of type commit not found: object not found - no match for id (69542c1984c1f9d91f7c6c9c9e6941782c944bd9); class=Odb (9); code=NotFound (-3)
+    Internal error: Failed to edit a commit: Current working-copy commit not found: Object 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 of type commit not found: An object with id 69542c1984c1f9d91f7c6c9c9e6941782c944bd9 could not be found
     "###);
 }