manifest_file must be an absolute path

Correctly pass the full path of the manifest file for the submanifest.
The manifest-name in the <submanifest/> element was being passed in
as given, which caused it to not be found since the current directory
never set. (b/226333721: fails when manifest-name is given.)

Also verify that the manifest_file passed to XmlManifest() is an
absolute path.

Bug: https://b.corp.google.com/issues/226333721
Change-Id: I23461078233e34562bc2eafeb732cfe8bd38ddc1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/333861
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>