Update precommit hooks to work in GitHub Desktop (#51404)

Turns out GitHub Desktop actually does bundle a sh.exe, just not a bash.

Also tries to look for the py launcher to help people who didn't check 
the "Add Python to PATH" option.

tools/hooks/python.sh

@@ -1,17 +1,22 @@
-#!/bin/bash
+#!/bin/sh
+# `sh` must be used here instead of `bash` to support GitHub Desktop.
 set -e
 if command -v python3 >/dev/null 2>&1; then
 	PY=python3
-else
+elif command -v python >/dev/null 2>&1; then
 	PY=python
+elif command -v py >/dev/null 2>&1; then
+	PY=py
+else
+	echo "Please install Python 3.6 or later."
 fi
 PATHSEP=$($PY - <<'EOF'
 import sys, os
 if sys.version_info.major != 3 or sys.version_info.minor < 6:
-	sys.stderr.write("Python 3.6+ is required: " + sys.version + "\n")
+	sys.stderr.write("Python 3.6 or later is required, but you have:\n" + sys.version + "\n")
 	exit(1)
 print(os.pathsep)
 EOF
 )
 export PYTHONPATH=tools/mapmerge2/${PATHSEP}${PYTHONPATH}
-$PY "$@"
+exec $PY "$@"