move UserAgent to git_command for wider user

We can't import the main module, so move the UserAgent helper out of
it and into the git_command module so it can be used in more places.

Bug: https://crbug.com/gerrit/11144
Change-Id: I8093c8a20bd1dc7d612d0e2a85180341817c0d86
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231057
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

tests/test_git_command.py

@@ -18,6 +18,7 @@
 
 from __future__ import print_function
 
+import re
 import unittest
 
 import git_command
@@ -47,3 +48,15 @@ class GitCallUnitTest(unittest.TestCase):
     self.assertLess(ver, (9999, 9999, 9999))
 
     self.assertNotEqual('', ver.full)
+
+
+class RepoUserAgentUnitTest(unittest.TestCase):
+  """Tests the RepoUserAgent function."""
+
+  def test_smoke(self):
+    """Make sure it returns something useful."""
+    ua = git_command.RepoUserAgent()
+    # We can't dive too deep because of OS/tool differences, but we can check
+    # the general form.
+    m = re.match(r'^git-repo/[^ ]+ ([^ ]+) git/[^ ]+ Python/[0-9.]+', ua)
+    self.assertIsNotNone(m)