Support main branches other than "master"

This patch adds support for having the main branch be named different
than "master".

It will use "master" or "main" if available, and fall back to the first
branch name if they're both missing.

git.py

@@ -223,6 +223,17 @@ class Repo:
         refs = self._for_each_ref(pattern="refs/heads/", sort="-authordate")
         return [ref[len("refs/heads/") :] for _, _, ref in refs]
 
+    @functools.cache
+    def main_branch(self):
+        """Get the name of the main branch."""
+        bs = self.branch_names()
+        for branch in ["master", "main"]:
+            if branch in bs:
+                return branch
+        if bs:
+            return bs[0]
+        return None
+
     @functools.cache
     def tags(self, sort="-taggerdate"):
         """Get the (name, obj_id) of the tags."""

views/summary.html

@@ -35,20 +35,22 @@
 <hr/>
 % end
 
-% if "master" in repo.branch_names():
+% if repo.main_branch():
 <div class="toggable-title" onclick="toggle('commits')">
-  <a href="b/master/">commits (master)</a>
+  <a href="b/{{repo.main_branch()}}/">commits ({{repo.main_branch()}})</a>
 </div>
-%     kwargs = dict(repo = repo, start_ref = "refs/heads/master",
+%     kwargs = dict(repo = repo,
+%                   start_ref = "refs/heads/" + repo.main_branch(),
 %                   limit = repo.info.commits_in_summary,
 %                   shorten = shorten, repo_root = ".", offset = 0)
 %     include commit-list **kwargs
 <hr/>
 <div class="toggable-title" onclick="toggle('ls')">
-  <a href="b/master/t/">tree (master)</a>
+  <a href="b/{{repo.main_branch()}}/t/">tree ({{repo.main_branch()}})</a>
 </div>
-%     kwargs = dict(repo = repo, tree=repo.tree("master"),
-%                   treeroot="b/master/t", dirname=smstr.from_url(""))
+%     kwargs = dict(repo = repo, tree=repo.tree(repo.main_branch()),
+%                   treeroot="b/" + repo.main_branch() + "/t",
+%                   dirname=smstr.from_url(""))
 %     include tree-list **kwargs
 <hr/>
 % end