Add a linear_fake_merge option (defaulting to True)

One of my projects (projectatomic/rpm-ostree) has a protected master
branch.  I was rather horrified to realize that homu force-pushes to
master by default.  Admittedly, I understand this is the only way to
make github think the PR was merged (as opposed to just closed).

But in my mind, that's a deficiency of github - it should have an API
to allow us to "close merged with this commit".

Anyways, add an option so that those who prefer closed over force
pushing can do that today.

We at least retitle the commit so that it's more clear it was merged.

cfg.sample.toml

@@ -44,6 +44,13 @@ try_users = []
 # Auto-squash commits. Requires the local Git command
 #autosquash = true
 
+# Whether homu should push a temporary merge commit that causes
+# github to think the PR was merged.  You need to disable this if
+# you have branch protection enabled.  Also, leaving this enabled
+# means that other people pulling master might transiently see
+# the merge commit.
+#linear_fake_merge = true
+
 ## branch names (these settings here are the defaults)
 #[repo.NAME.branch]
 #

homu/main.py

@@ -214,7 +214,12 @@ def refresh(self):
         self.body = issue.body
 
     def fake_merge(self, repo_cfg):
-        if repo_cfg.get('linear', False) or repo_cfg.get('autosquash', False):
+        is_linear_or_autosquash = repo_cfg.get('linear', False) or repo_cfg.get('autosquash', False)
+        # Backwards compat, though I think this is a terrible default.
+        fake_merge = repo_cfg.get('linear_fake_merge', True)
+        if not is_linear_or_autosquash:
+            return
+        if fake_merge:
             msg = '''!!! Temporary commit !!!
 
 This commit is artifically made up to mark PR {} as merged.
@@ -238,6 +243,16 @@ def fail(err):
                 self.get_issue().close()
 
             utils.retry_until(inner, fail, self)
+        else:
+            issue = self.get_issue()
+            title = issue.title
+            # So unfortunately, we can't yet tell github this PR was
+            # merged.  We need to rework things so that our last
+            # comment at least includes a link to the master commit.
+            merged_prefix = '[merged] '
+            if not title.startswith(merged_prefix):
+                title = merged_prefix + title
+            issue.edit(title=title, state='closed')
 
 def sha_cmp(short, full):
     return len(short) >= 4 and short == full[:len(short)]