Simplify docs build

Remove unnecessary old code from tools/build_docs.sh; it now only builds
docs for the current branch in the current working directory.

Fix the merging if the git log changes into changes.html.

Change-Id: I965dc3383b6317807ca0d47fe42648a19e96b57b
diff --git a/doc/source/changes.rst b/doc/source/changes.rst
index f4a326d..abe132c 100644
--- a/doc/source/changes.rst
+++ b/doc/source/changes.rst
@@ -8,3 +8,5 @@
 These are the commits to DevStack for the last six months. For the
 complete list see `the DevStack project in
 Gerrit <https://review.openstack.org/#/q/status:merged+project:openstack-dev/devstack,n,z>`__.
+
+%GIT_LOG%