Use the new section title style in Asciidoctor.

We previous use the section title style like:

Section level 1
===============

Section level 2
---------------

Which have a problem in Asciidoctor that the number of "="s or "-"s must match
the number of characters in the header exactly, as a result it's easy to make
mistakes while changing the titles. Asciidoctor provides a better style like:

= Section level 1

== Section level 2

So we switched to this style.

Also fixed a bug in replace_macros.py, which will not cause any problem in the
old style.

Change-Id: I811dd7238735d98f662767c17086152cd69aea02
diff --git a/Documentation/error-commit-already-exists.txt b/Documentation/error-commit-already-exists.txt
index 6a2170a..f90ae42 100644
--- a/Documentation/error-commit-already-exists.txt
+++ b/Documentation/error-commit-already-exists.txt
@@ -1,5 +1,4 @@
-commit already exists
-=====================
+= commit already exists
 
 With this error message Gerrit rejects to push a commit to an
 existing change via `refs/changes/n` if the commit was already