Merge ~mwhudson/germinate:clone-depth-1 into germinate:master

[Michael Hudson-Doyle]

perform shallow clones of seed repos

The repos are not very large to start with but it does make a noticeable
difference for me (8s vs 5s or so)

[Colin Watson (cjwatson)]

review: Approve

diff --git a/germinate/seeds.py b/germinate/seeds.py
index 7752b16..fe15734 100644
--- a/germinate/seeds.py
+++ b/germinate/seeds.py
@@ -148,7 +148,7 @@ class Seed:
             path = os.path.join(base, repository)
             if not path.endswith("/"):
                 path += "/"
-            command = ["git", "clone"]
+            command = ["git", "clone", "--depth", "1"]
             if git_branch is not None:
                 command.extend(["-b", git_branch])
                 _logger.info("Cloning branch %s of %s", git_branch, path)