[git_cl] Extract construction inference

Potentially makes it easier to test (needs more deps extracted and
injected though).

Change-Id: I8c551b39c97ec95291995e614c760073b10a4615
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5689682
Commit-Queue: Allen Li <ayatane@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -3666,30 +3666,51 @@ def DownloadGerritHook(force):
 
 def ConfigureGitRepoAuth() -> None:
     """Configure the current Git repo authentication."""
-    c = GitAuthConfigChanger()
+    c = GitAuthConfigChanger.infer_and_create()
     c.apply()
 
 
 class GitAuthConfigChanger(object):
     """Changes Git auth config as needed for Gerrit."""
 
-    def __init__(self):
-        cl = Changelist()
+    def __init__(
+        self,
+        *,
+        host_shortname: str,
+        remote_url: str,
+    ):
+        """Create a new GitAuthConfigChanger.
 
+        Args:
+            host_shortname: Gerrit host shortname, e.g., chromium
+            remote_url: Git repository's remote URL, e.g.,
+                https://chromium.googlesource.com/chromium/tools/depot_tools.git
+        """
+        self._shortname: str = host_shortname
+        parts: urllib.parse.SplitResult = urllib.parse.urlsplit(remote_url)
+        # Base URL looks like https://chromium.googlesource.com/
+        self._base_url: str = parts._replace(path='/', query='',
+                                             fragment='').geturl()
+
+        self._should_use_sso: bool = gerrit_util.ShouldUseSSO(gerrit_host)
+
+    @classmethod
+    def infer_and_create(cls) -> 'GitAuthConfigChanger':
+        """Create a GitAuthConfigChanger by inferring from env."""
+        cl = Changelist()
         # chromium-review.googlesource.com
         gerrit_host: str = cl.GetGerritHost()
         # chromium
-        self._shortname: str = gerrit_host.split('.')[0][:-len('-review')]
+        host_shortname: str = gerrit_host.split('.')[0][:-len('-review')]
 
         # These depend on what the user set for their remote
         # https://chromium.googlesource.com/chromium/tools/depot_tools.git
         remote_url: str = cl.GetRemoteUrl()
-        parts: urllib.parse.SplitResult = urllib.parse.urlsplit(remote_url)
-        # https://chromium.googlesource.com/
-        self._base_url: str = parts._replace(path='/', query='',
-                                             fragment='').geturl()
 
-        self._should_use_sso: bool = gerrit_util.ShouldUseSSO(gerrit_host)
+        return cls(
+            host_shortname=host_shortname,
+            remote_url=remote_url,
+        )
 
     def apply(self):
         """Apply config changes."""