Handle repositories with a default branch other than master

This commit adds a new configuration option `default_branch`, which is
used to specify the default/base branch a repository is using.
Previously reno assumed the default branch was always named 'master'
which causes commands such as 'reno lint' to fail when it is run on a
repository that uses a different default branch name such as 'main'.
This config option gives users an option to specify which branch should
be used for cases like that.

Change-Id: I08f258206290fdc1dcb9fcfc078e30384667c925

reno/config.py

@@ -57,6 +57,13 @@ _OPTIONS = [
         would be converted to ``liberty-eol``.
         """)),
 
+    Opt('default_branch', 'master',
+        textwrap.dedent("""\
+        The default git branch for the repository. This is the base branch that
+        is treated as the root for other branches. By default this is
+        ``master``.
+        """)),
+
     Opt('earliest_version', None,
         textwrap.dedent("""\
         The earliest version to be included. This is usually the

reno/scanner.py

@@ -662,7 +662,7 @@ class Scanner(object):
         # branch until we find something that is on both.
         master_commits = set(
             c.commit.sha().hexdigest()
-            for c in self._get_walker_for_branch('master')
+            for c in self._get_walker_for_branch(self.conf.default_branch)
         )
         for c in self._get_walker_for_branch(branch):
             if c.commit.sha().hexdigest() in master_commits:
@@ -912,7 +912,7 @@ class Scanner(object):
             # scan.
             return None
         # We need to look for the previous branch's root.
-        if branch and branch != 'master':
+        if branch and branch != self.conf.default_branch:
             previous_branch = self._get_earlier_branch(branch)
             if not previous_branch:
                 # This was the first branch, so scan the whole