manifest_xml: refactor manifest parsing from client management

We conflate the manifest & parsing logic with the management of the
repo client checkout in a single class.  This makes testing just one
part (the manifest parsing) hard as it requires a full checkout too.

Start splitting the two apart into separate classes to make it easy
to reason about & test.

Change-Id: Iaf897c93db9c724baba6044bfe7a589c024523b2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/288682
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -510,7 +510,7 @@ class Project(object):
                      with exponential backoff and jitter.
       old_revision: saved git commit id for open GITC projects.
     """
-    self.manifest = manifest
+    self.client = self.manifest = manifest
     self.name = name
     self.remote = remote
     self.gitdir = gitdir.replace('\\', '/')
@@ -551,7 +551,7 @@ class Project(object):
     self.linkfiles = []
     self.annotations = []
     self.config = GitConfig.ForRepository(gitdir=self.gitdir,
-                                          defaults=self.manifest.globalConfig)
+                                          defaults=self.client.globalConfig)
 
     if self.worktree:
       self.work_git = self._GitGetByExec(self, bare=False, gitdir=gitdir)
@@ -1168,7 +1168,7 @@ class Project(object):
     self._InitHooks()
 
   def _CopyAndLinkFiles(self):
-    if self.manifest.isGitcClient:
+    if self.client.isGitcClient:
       return
     for copyfile in self.copyfiles:
       copyfile._Copy()