extension to print commit message that would be supplied to editor

Summary:
the nuclide team would like to be able to get the commit template
message without actually invoking a commit. This small extensions allows them
to do this.

Caveats: ignores subrepositories.

Test Plan: new test

Reviewers: #mercurial, most

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D3960557

Tasks: 12771006

hgext3rd/debugcommitmessage.py

@@ -0,0 +1,43 @@
+from mercurial import (
+    cmdutil,
+    context,
+    error,
+)
+from mercurial.i18n import _
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+@command('debugcommitmessage', [], _('FORM'))
+def debugcommitmessage(ui, repo, *args):
+    form = None
+    if len(args) > 1:
+        raise error.Abort(_('provide at most one form'))
+    elif len(args) > 0:
+        form = args[0]
+
+    status = repo.status()
+    text = ''
+    user = None
+    date = None
+    extra = None
+
+    ctx = context.workingcommitctx(repo, status, text, user, date, extra)
+
+    subs = []
+    editform = form or 'commit.normal.normal'
+    extramsg = _('Leave message empty to abort commit.')
+
+    forms = [e for e in editform.split('.') if e]
+    forms.insert(0, 'changeset')
+    while forms:
+        tmpl = repo.ui.config('committemplate', '.'.join(forms))
+        if tmpl:
+            committext = cmdutil.buildcommittemplate(
+                    repo, ctx, subs, extramsg, tmpl)
+            break
+        forms.pop()
+    else:
+        committext = cmdutil.buildcommittext(repo, ctx, subs, extramsg)
+
+    ui.status(committext)

tests/test-debugcommitmessage.t

@@ -0,0 +1,35 @@
+Set up extension
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > debugcommitmessage = $TESTDIR/../hgext3rd/debugcommitmessage.py
+  > EOF
+
+Set up repo
+  $ hg init repo
+  $ cd repo
+
+Test extension
+  $ hg debugcommitmessage
+  
+  
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: test
+  HG: branch 'default'
+  HG: no files changed
+  $ hg debugcommitmessage --config committemplate.changeset.commit.normal.normal="Test Specific Message\n"
+  Test Specific Message
+  $ hg debugcommitmessage --config committemplate.changeset.commit="Test Generic Message\n"
+  Test Generic Message
+  $ hg debugcommitmessage commit.amend.normal --config committemplate.changeset.commit="Test Generic Message\n"
+  Test Generic Message
+  $ hg debugcommitmessage randomform --config committemplate.changeset.commit="Test Generic Message\n"
+  
+  
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: test
+  HG: branch 'default'
+  HG: no files changed