scm: unified progress reporting in a structured way

This adds a support for reporting progress from Git in a structured
format. The gitpython already provides somewhat structured progress
reporting, dulwich on the other hand only provides git-like line
messages.

As gitpython uses git binary, it already parse this line format
in a structured way. This PR makes dulwich use the same machinery
to unify progress reporting for both dulwich and gitpython python.

This does add a dependency on gitpython even on dulwich backend,
so I am iffy if we should do this.
The alternative is to translate gitpython's progress reports to
string, and just use string messages as progress reports on both
dulwich and gitpython.

tests/func/test_external_repo.py

@@ -140,7 +140,9 @@ def test_shallow_clone_branch(erepo_dir):
             with repo.open_by_relpath("file") as fd:
                 assert fd.read() == "branch"
 
-        mock_clone.assert_called_with(url, ANY, shallow_branch="branch")
+        mock_clone.assert_called_with(
+            url, ANY, shallow_branch="branch", progress=ANY
+        )
         _, shallow = CLONES[url]
         assert shallow
 
@@ -166,7 +168,9 @@ def test_shallow_clone_tag(erepo_dir):
             with repo.open_by_relpath("file") as fd:
                 assert fd.read() == "foo"
 
-        mock_clone.assert_called_with(url, ANY, shallow_branch="v1")
+        mock_clone.assert_called_with(
+            url, ANY, shallow_branch="v1", progress=ANY
+        )
         _, shallow = CLONES[url]
         assert shallow