faq: mercurial vs. git

Fixes #4052.

R=golang-dev, bradfitz, iant, rsc
CC=golang-dev
https://golang.org/cl/6490092

doc/go_faq.html

@@ -922,6 +922,32 @@ See the document
 for more information about how to proceed.
 </p>
 
+<h3 id="Why_does_the_project_use_Mercurial_and_not_git">
+Why does the project use Mercurial and not git?</h3>
+
+<p>
+The Go project, hosted by Google Code at
+<a href="http://code.google.com/p/go">code.google.com/p/go</a>,
+uses Mercurial as its version control system.
+When the project launched,
+Google Code supported only Subversion and Mercurial.
+Mercurial was a better choice because of its plugin mechanism
+that allowed us to create the "codereview" plugin to connect
+the project to the excellent code review tools at 
+<a href="http://codereview.appspot.com">codereview.appspot.com</a>.
+</p>
+
+<p>
+Programmers who work
+with the Go project's source rather than release downloads sometimes
+ask for the project to switch to git.
+That would be possible, but it would be a lot of work and
+would also require reimplementing the codereview plugin.
+Given that Mercurial works today, with code review support,
+combined with the Go project's mostly linear, non-branching use of
+version control, a switch to git doesn't seem worthwhile.
+</p>
+
 <h2 id="Pointers">Pointers and Allocation</h2>
 
 <h3 id="pass_by_value">