amend: save commit message into ".hg/last-message.txt"

Before this patch, commit message (may be manually edited) for "commit
--amend" is never saved into ".hg/last-message.txt", because it uses
"localrepository.commitctx()" instead of "localrepository.commit()":
saving into ".hg/last-message.txt" is executed only in the latter.

This patch saves commit message for "commit --amend" into
".hg/last-message.txt" just after user editing.

This is the simplest implementation to fix on stable. Editing and
saving commit message for memctx should be centralized into the
framework like "localrepository.commit()" with "editor" argument or so
in the future.

mercurial/cmdutil.py

@@ -1782,6 +1782,7 b' def amend(ui, repo, commitfunc, old, ext'
                                  extra=extra)
             if editmsg:
                 new._text = commitforceeditor(repo, new, [])
+            repo.savecommitmessage(new.description())
 
             newdesc =  changelog.stripdesc(new.description())
             if ((not node)

tests/test-commit-amend.t

@@ -165,6 +165,57 b' Open editor with old commit message if a'
   > cat $1
   > echo "another precious commit message" > "$1"
   > __EOF__
+
+at first, test saving last-message.txt
+
+  $ cat > .hg/hgrc << '__EOF__'
+  > [hooks]
+  > pretxncommit.test-saving-last-message = false
+  > __EOF__
+
+  $ rm -f .hg/last-message.txt
+  $ hg commit --amend -v -m "message given from command line"
+  amending changeset 5f357c7560ab
+  copying changeset 5f357c7560ab to ad120869acf0
+  a
+  running hook pretxncommit.test-saving-last-message: false
+  transaction abort!
+  rollback completed
+  abort: pretxncommit.test-saving-last-message hook exited with status 1
+  [255]
+  $ cat .hg/last-message.txt
+  message given from command line (no-eol)
+
+  $ rm -f .hg/last-message.txt
+  $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg commit --amend -v
+  amending changeset 5f357c7560ab
+  copying changeset 5f357c7560ab to ad120869acf0
+  no changes, new message
+  
+  
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: foo
+  HG: branch 'default'
+  HG: changed a
+  a
+  running hook pretxncommit.test-saving-last-message: false
+  transaction abort!
+  rollback completed
+  abort: pretxncommit.test-saving-last-message hook exited with status 1
+  [255]
+
+  $ cat .hg/last-message.txt
+  another precious commit message
+
+  $ cat > .hg/hgrc << '__EOF__'
+  > [hooks]
+  > pretxncommit.test-saving-last-message =
+  > __EOF__
+
+then, test editing custom commit message
+
   $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg commit --amend -v
   amending changeset 5f357c7560ab
   copying changeset 5f357c7560ab to ad120869acf0