path: extract sub-option logic into its own method
We will need to re-use this logic for `path://` so we first extract it into its own method.
Differential Revision: https://phab.mercurial-scm.org/D10262
## What is it?
This extension adds ability to save certain pushes to a remote blob store
as bundles and to serve commits from remote blob store.
The revisions are stored on disk or in everstore.
The metadata are stored in sql or on disk.
## Config options
infinitepush.branchpattern: pattern to detect a scratchbranch, example
're:scratch/.+'
infinitepush.indextype: disk or sql for the metadata
infinitepush.reponame: only relevant for sql metadata backend, reponame to put in
sql
infinitepush.indexpath: only relevant for ondisk metadata backend, the path to
store the index on disk. If not set will be under .hg
in a folder named filebundlestore
infinitepush.storepath: only relevant for ondisk metadata backend, the path to
store the bundles. If not set, it will be
.hg/filebundlestore