repo: add support for reading standalone manifests from disk

BUG=b:192664812
TEST=existing tests (no coverage), manual runs

Change-Id: Ic032417ecfca77d5e0de1b1ff62b30ce8205bfc5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/318715
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

fetch.py

@@ -35,4 +35,7 @@ def fetch_file(url):
       print('fatal: error running "gsutil": %s' % e.output,
             file=sys.stderr)
     sys.exit(1)
+  if scheme == 'file':
+    with open(url[len('file://'):], 'rb') as f:
+      return f.read()
   raise ValueError('unsupported url %s' % url)