git: abort when attempting to set a branch

Given the mapping we use (namely, a git head is a bookmark), it is better to
error out with a hint.

hgext/git/dirstate.py

@@ -300,3 +300,8 b' class gitdirstate(object):'
     def clearbackup(self, tr, backupname):
         # TODO
         pass
+
+    def setbranch(self, branch):
+        raise error.Abort(
+            b'git repos do not support branches. try using bookmarks'
+        )