shelve: move actual created commit shelving to a separate function
Currently, this code does not have any branching, it just bundles
a commit and saves a patch file. Later, obsolescence-based shelve
will be added, so this code will also create some obsmarkers and
will be one of the few places where obsshelve will be different
from traditional shelve.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).