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
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https://www.mercurial-scm.org/wiki/ContributingChanges
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273ce12ad8f1` will report some style violations on a very old commit).