magit-push-hook: make the default value a list

This allows adding hook functions using `magit-add-section-hook'.
(Yes, this isn't a section hook, but besides being the disadvantage of
not supporting hooks that have a non-nil value, this function also has
advantages over `add-hook', mainly more control over where the new entry
is inserted.
stgit: Fix 'magit-run-stgit'

Use 'magit-call-process' instead of 'magit-call-git'.
Merge pull request #1151 from llvilanova/magit-run-stgit

stgit: Fix 'magit-run-stgit'
stgit: Do not propertize patch message

Face 'magit-stgit' does not exist.
stgit: Allow empty patch messages
Merge branch 'pr/stgit-series-ui'