rebase: only store collapse message once
The message is determined by the user passing --message or --log when
the rebase is started. There's no need to write it to a file for each
rebased commit; writing it once at the start of the rebase is enough.
Differential Revision: https://phab.mercurial-scm.org/D2712
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}