rebase: use hard-coded template for one-line commit description
This is to prepare for making making the one-line summary
customizable. The template ended up pretty complicated because of the
conditional output of "(<bookmarks etc>)". Maybe we can simplify the
template later.
Differential Revision: https://phab.mercurial-scm.org/D9250
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h