Revert "Deprecates RUN_THIS.py in favor of manually doin' it (#3930)" (#3933)

This reverts commit 94082e07a7.

BuildChecker/hooks/post-checkout

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+gitroot=`git rev-parse --show-toplevel`
+
+cd "$gitroot/BuildChecker"
+
+if [[ `uname` == MINGW* || `uname` == CYGWIN* ]]; then
+    # Windows
+    py -3 git_helper.py --quiet
+else
+    # Not Windows, so probably some other Unix thing.
+    python3 git_helper.py --quiet
+fi

BuildChecker/hooks/post-merge

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# Just call post-checkout since it does the same thing.
+gitroot=`git rev-parse --show-toplevel`
+bash "$gitroot/.git/hooks/post-checkout"