[FIX] runbot_merge: avoid editing title line of commits

When a commit is lacking the purpose (?) tag e.g. `[FIX]`, `[IMP]`,
..., a normal commit message of the form `<module>: <info>` marches
the looks of a git pseudo-header.

This results in a commit rewrite rejiggering the entire thing and
breaking the message by moving the title to the pseudo-headers and
mis-promoting either the `closes` line of body content to "title",
resulting in a really crappy commit message
e.g. odoo/odoo@d4aa9ad031.

Update the commit rewriting procedure to specifically skip the title
line, and re-inject it without processing in the output.

Fixes #540

runbot_merge/models/pull_requests.py

@@ -2232,7 +2232,9 @@ class Message:
         msg, handle_break = (msg, False) if isinstance(msg, str) else (msg.message, True)
         headers = []
         body = []
-        for line in reversed(msg.splitlines()):
+        # don't process the title (first line) of the commit message
+        msg = msg.splitlines()
+        for line in reversed(msg[1:]):
             if maybe_setex:
                 # NOTE: actually slightly more complicated: it's a SETEX heading
                 #       only if preceding line(s) can be interpreted as a
@@ -2268,6 +2270,10 @@ class Message:
             body.append(line)
             in_headers = False
 
+        # if there are non-title body lines, add a separation after the title
+        if body and body[-1]:
+            body.append('')
+        body.append(msg[0])
         return cls('\n'.join(reversed(body)), Headers(reversed(headers)))
 
     def __init__(self, body, headers=None):

runbot_merge/tests/test_basic.py

@@ -1803,7 +1803,7 @@ removed
 
             repo.make_commits(root, Commit('Commit\n\nfirst\n***\nsecond', tree={'a': 'b'}), ref=f'heads/change')
             pr = repo.make_pr(title="PR", body=f'first\n***\nsecond',
-                              target='master', head=f'change')
+                              target='master', head='change')
             repo.post_status(pr.head, 'success', 'legal/cla')
             repo.post_status(pr.head, 'success', 'ci/runbot')
             pr.post_comment('hansen r+', config['role_reviewer']['token'])
@@ -1827,6 +1827,44 @@ removed
         Signed-off-by: {reviewer}
         """).strip(), "squashed / rebased messages should not be stripped"
 
+    def test_title_no_edit(self, repo, env, users, config):
+        """The first line of a commit message should not be taken in account for
+        rewriting, especially as it can be untagged and interpreted as a
+        pseudo-header
+        """
+        with repo:
+            repo.make_commits(None, Commit("0", tree={'a': '1'}), ref='heads/master')
+            repo.make_commits(
+                'master',
+                Commit('Some: thing\n\nis odd', tree={'b': '1'}),
+                Commit('thing: thong', tree={'b': '2'}),
+                ref='heads/change')
+
+            pr = repo.make_pr(target='master', head='change')
+            repo.post_status(pr.head, 'success', 'legal/cla')
+            repo.post_status(pr.head, 'success', 'ci/runbot')
+            pr.post_comment('hansen rebase-ff r+', config['role_reviewer']['token'])
+        env.run_crons()
+
+        pr_id = to_pr(env, pr)
+        assert pr_id.staging_id # check PR is staged
+
+
+        reviewer = get_partner(env, users["reviewer"]).formatted_email
+        staging_head = repo.commit('staging.master')
+        assert staging_head.message == f"""\
+thing: thong
+
+closes {pr_id.display_name}
+
+Signed-off-by: {reviewer}"""
+        assert repo.commit(staging_head.parents[0]).message == f"""\
+Some: thing
+
+is odd
+
+Part-of: {pr_id.display_name}"""
+
     def test_pr_mergehead(self, repo, env, config):
         """ if the head of the PR is a merge commit and one of the parents is
         in the target, replicate the merge commit instead of merging