add source code links/licenses/languages for version control systems

- use https URLs for all links

README.md

@@ -654,11 +654,11 @@ Troubleshooting tools.
 
 Software versioning and revision control.
 
-- [Darcs](http://darcs.net/) - Cross-platform version control system, like git, mercurial or svn but with a very different approach: focus on changes rather than snapshots. ([Source Code](https://darcs.net/releases/)) `GPL-2.0` `Haskell`
-- [Fossil](http://www.fossil-scm.org/) - Distributed version control with built-in wiki and bug tracking. ([Source Code](https://www.fossil-scm.org/home/dir?ci=trunk)) `BSD-2-Clause` `C`
-- [Git](http://git-scm.com/) - Distributed revision control and source code management (SCM) with an emphasis on speed. ([Source Code](https://github.com/git/git)) `GPL-2.0` `C`
-- [Mercurial](https://www.mercurial-scm.org/) - Another distributed revision control.
-- [Subversion](http://subversion.apache.org/) - Client-server revision control system.
+- [Darcs](https://darcs.net/) - Cross-platform version control system, like git, mercurial or svn but with a very different approach: focus on changes rather than snapshots. ([Source Code](https://darcs.net/releases/)) `GPL-2.0` `Haskell`
+- [Fossil](https://www.fossil-scm.org/) - Distributed version control with built-in wiki and bug tracking. ([Source Code](https://www.fossil-scm.org/home/dir?ci=trunk)) `BSD-2-Clause` `C`
+- [Git](https://git-scm.com/) - Distributed revision control and source code management (SCM) with an emphasis on speed. ([Source Code](https://github.com/git/git)) `GPL-2.0` `C`
+- [Mercurial](https://www.mercurial-scm.org/) - Distributed source control management tool. ([Source Code](https://repo.mercurial-scm.org/hg/file/tip)) `GPL-2.0` `Python/C/Rust`
+- [Subversion](https://subversion.apache.org/) - Client-server revision control system.([Source Code](https://svn.apache.org/repos/asf/subversion/trunk/)) `Apache-2.0` `C`
 
 
 ### Virtualization