Deprecate SDL_GetRevisionNumber and update things for git instead of hg.

Fixes #4063

src/SDL.c

@@ -468,7 +468,7 @@ SDL_GetRevision(void)
 int
 SDL_GetRevisionNumber(void)
 {
-    return SDL_REVISION_NUMBER;
+    return 0;  /* doesn't make sense without Mercurial. */
 }
 
 /* Get the name of the platform */

src/dynapi/gendynapi.pl

@@ -21,7 +21,7 @@
 
 # WHAT IS THIS?
 #  When you add a public API to SDL, please run this script, make sure the
-#  output looks sane (hg diff, it adds to existing files), and commit it.
+#  output looks sane (git diff, it adds to existing files), and commit it.
 #  It keeps the dynamic API jump table operating correctly.
 
 # If you wanted this to be readable, you shouldn't have used perl.