amend: use an explicit commit message for temporary amending commit

Before this changeset, the extra commit created during amend had
the same description as the final commit. This was a bit confusing
when trying to understand what that extra commit was about.

This changeset changes the description of such commit to:

    temporary amend commit for <ammend-commit-hash>

The old behaviour was not a big deal, but would become more confusing
once we use obsolescence marker instead of stripping the precursors.

This also helps if the user restores a strip backup.

mercurial/cmdutil.py

@@ -1582,10 +1582,14 b' def amend(ui, repo, commitfunc, old, ext'
         lock = repo.lock()
         tr = repo.transaction('amend')
         try:
+            # See if we got a message from -m or -l, if not, open the editor
+            # with the message of the changeset to amend
+            message = logmessage(ui, opts)
             # First, do a regular commit to record all changes in the working
             # directory (if there are any)
             ui.callhooks = False
             try:
+                opts['message'] = 'temporary amend commit for %s' % old
                 node = commit(ui, repo, commitfunc, pats, opts)
             finally:
                 ui.callhooks = True
@@ -1618,7 +1622,6 b' def amend(ui, repo, commitfunc, old, ext'
 
                 user = ctx.user()
                 date = ctx.date()
-                message = ctx.description()
                 # Recompute copies (avoid recording a -> b -> a)
                 copied = copies.pathcopies(base, ctx)
 
@@ -1663,17 +1666,10 b' def amend(ui, repo, commitfunc, old, ext'
                     except KeyError:
                         raise IOError
 
-                # See if we got a message from -m or -l, if not, open the editor
-                # with the message of the changeset to amend
                 user = opts.get('user') or old.user()
                 date = opts.get('date') or old.date()
-                message = logmessage(ui, opts)
-                if not message:
-                    cctx = context.workingctx(repo, old.description(),
-                                              user, date, extra,
-                                              repo.status(base.node(),
-                                              old.node()))
-                    message = commitforceeditor(repo, cctx, [])
+            if not message:
+                message = old.description()
 
             new = context.memctx(repo,
                                  parents=[base.node(), nullid],
@@ -1683,6 +1679,7 b' def amend(ui, repo, commitfunc, old, ext'
                                  user=user,
                                  date=date,
                                  extra=extra)
+            new._text = commitforceeditor(repo, new, [])
             ph = repo.ui.config('phases', 'new-commit', phases.draft)
             try:
                 repo.ui.setconfig('phases', 'new-commit', old.phase())

tests/test-commit-amend.t

@@ -167,6 +167,8 b' Same, but with changes in working dir (d'
   $ echo a >> a
   $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg commit --amend -v
   amending changeset ffb49186f961
+  a
+  copying changeset a4f8a65b7c6a to ad120869acf0
   another precious commit message
   
   
@@ -177,9 +179,7 b' Same, but with changes in working dir (d'
   HG: branch 'default'
   HG: changed a
   a
-  copying changeset 27f3aacd3011 to ad120869acf0
-  a
-  stripping intermediate changeset 27f3aacd3011
+  stripping intermediate changeset a4f8a65b7c6a
   stripping amended changeset ffb49186f961
   2 changesets found
   saved backup bundle to $TESTTMP/.hg/strip-backup/ffb49186f961-amend-backup.hg (glob)