init: use the remote default manifest branch

Instead of hardcoding "master" as our default, use the remote server's
default branch instead.  For most people, this should be the same as
"master" already.  For projects moving to "main", it means we'll use
the new name automatically rather than forcing people to use -b main.

For repositories that never set up a default HEAD, we should still use
the historical "master" default.

Bug: https://crbug.com/gerrit/13339
Change-Id: I4117c81a760c9495f98dbb1111a3e6c127f45eba
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/280799
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -2311,6 +2311,27 @@ class Project(object):
     # Enable the extension!
     self.config.SetString('extensions.%s' % (key,), value)
 
+  def ResolveRemoteHead(self, name=None):
+    """Find out what the default branch (HEAD) points to.
+
+    Normally this points to refs/heads/master, but projects are moving to main.
+    Support whatever the server uses rather than hardcoding "master" ourselves.
+    """
+    if name is None:
+      name = self.remote.name
+
+    # The output will look like (NB: tabs are separators):
+    # ref: refs/heads/master	HEAD
+    # 5f6803b100bb3cd0f534e96e88c91373e8ed1c44	HEAD
+    output = self.bare_git.ls_remote('-q', '--symref', '--exit-code', name, 'HEAD')
+
+    for line in output.splitlines():
+      lhs, rhs = line.split('\t', 1)
+      if rhs == 'HEAD' and lhs.startswith('ref:'):
+        return lhs[4:].strip()
+
+    return None
+
   def _CheckForImmutableRevision(self):
     try:
       # if revision (sha or tag) is not present then following function