manifest: relax include name rules for user-specified path

Allow the user to specify relative or absolute or any other funky
path that they want when using `repo init` or `repo sync`.  Our
goal is to restrict the paths in the remote manifest git repo we
cloned from the network, not protect the user from themselves.

Bug: https://crbug.com/gerrit/14156
Change-Id: I1ccfb2a6bd1dce2bd765e261bef0bbf0f8a9beb6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298823


Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>