chore: move Mercurial resources. books 2 subjects

handles proposal(lists): Merge Version Control Systems into same section EbookFoundation#5526

books/free-programming-books-langs.md

@@ -133,7 +133,6 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Mathematica](#mathematica)
 * [MATLAB](#matlab)
 * [Maven](#maven)
-* [Mercurial](#mercurial)
 * [Mercury](#mercury)
 * [Modelica](#modelica)
 * [MongoDB](#mongodb)
@@ -1513,13 +1512,6 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Repository Management with Nexus](http://books.sonatype.com/nexus-book/reference/)
 
 
-### Mercurial
-
-* [Hg Init: a Mercurial Tutorial](https://hginit.github.io) - Joel Spolsky
-* [Mercurial: The Definitive Guide](http://hgbook.red-bean.com)
-* [Mercurial: The Definitive Guide 2nd edition](https://book.mercurial-scm.org)
-
-
 ### Mercury
 
 * [The Mercury Users' Guide](http://www.mercurylang.org/information/doc-release/user_guide.pdf) (PDF)

books/free-programming-books-subjects.md

@@ -837,6 +837,9 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Pro Git Reedited](https://leanpub.com/progitreedited/read) - Jon Forrest
 * [Ry's Git Tutorial](https://web.archive.org/web/20161121145226/http://rypress.com:80/tutorials/git/index) - Ryan Hodson
 * [Think Like (a) Git: A Guide for the Perplexed](http://think-like-a-git.net)
+* [Hg Init: a Mercurial Tutorial](https://hginit.github.io) - Joel Spolsky
+* [Mercurial: The Definitive Guide](http://hgbook.red-bean.com)
+* [Mercurial: The Definitive Guide 2nd edition](https://book.mercurial-scm.org)
 
 
 ### Web Performance