Deprecate SDL_GetRevisionNumber and update things for git instead of hg.

Fixes #4063

docs/README-hg.md

@@ -1,6 +1,4 @@
-We are no longer hosted in Mercurial. The official repo is now:
-
-   https://github.com/libsdl-org/SDL
+We are no longer hosted in Mercurial. Please see README-git.md for details.
 
 Thanks!
 

docs/README-linux.md

@@ -15,7 +15,7 @@ Build Dependencies
     
 Ubuntu 20.04, all available features enabled:
 
-sudo apt-get install build-essential mercurial make cmake autoconf automake \
+sudo apt-get install build-essential git make cmake autoconf automake \
 libtool libasound2-dev libpulse-dev libaudio-dev libx11-dev libxext-dev \
 libxrandr-dev libxcursor-dev libxi-dev libxinerama-dev libxxf86vm-dev \
 libxss-dev libgl1-mesa-dev libdbus-1-dev libudev-dev libgles2-mesa-dev \

docs/README.md

@@ -35,7 +35,7 @@ More documentation and FAQs are available online at [the wiki](http://wiki.libsd
 - [DynAPI](README-dynapi.md)
 - [Emscripten](README-emscripten.md)
 - [Gesture](README-gesture.md)
-- [Mercurial](README-hg.md)
+- [Git](README-git.md)
 - [iOS](README-ios.md)
 - [Linux](README-linux.md)
 - [OS X](README-macosx.md)