Provide Repository.parent_revs() and Repository.children_revs() f…

…or `ticket-1492-svgweb`

With this change and together with rblank's graph view implementation
we get a working git commit graph view.

(See http://trac.edgewall.org/ticket/1492 for discussion)

tracext/git/git_fs.py

@@ -440,6 +440,12 @@ def next_rev(self, rev, path=''):
     def previous_rev(self, rev, path=''):
         return self.git.hist_prev_revision(rev)
 
+    def parent_revs(self, rev):
+        return self.git.parents(rev)
+
+    def child_revs(self, rev):
+        return self.git.children(rev)
+
     def rev_older_than(self, rev1, rev2):
         rc = self.git.rev_is_anchestor_of(rev1, rev2)
         return rc