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>

subcmds/info.py

@@ -44,7 +44,7 @@ class Info(PagedCommand):
                  help="Disable all remote operations")
 
   def Execute(self, opt, args):
-    self.out = _Coloring(self.manifest.globalConfig)
+    self.out = _Coloring(self.client.globalConfig)
     self.heading = self.out.printer('heading', attr='bold')
     self.headtext = self.out.nofmt_printer('headtext', fg='yellow')
     self.redtext = self.out.printer('redtext', fg='red')