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

hgext/rebase.py

@@ -478,7 +478,6 b' class rebaseruntime(object):'
                                              self.state, self.skipped,
                                              self.obsoletenotrebased)
                 self.storestatus(tr=tr)
-                storecollapsemsg(repo, self.collapsemsg)
                 if len(repo[None].parents()) == 2:
                     repo.ui.debug('resuming interrupted rebase\n')
                 else:
@@ -846,6 +845,7 b' def _origrebase(ui, repo, inmemory=False'
             retcode = rbsrt._preparenewrebase(destmap)
             if retcode is not None:
                 return retcode
+            storecollapsemsg(repo, rbsrt.collapsemsg)
 
         tr = None