refs #4472. Corrected problem with detecting executable file.

If you tried to commit a file with the executable flag set that
wasn't one of the normal extensions it would look at the first
line to see if it could see the normal unix #!/blah.

But the "!" in the grep wasn't escaped - so it is now!

.githooks/pre-commit

@@ -121,7 +121,7 @@ mode_looks_exe() {
 		*.exe) return 0 ;;
 		*.com) return 0 ;;
 	esac
-	git cat-file blob "$2" | head -1 | grep "^#!/" > /dev/null
+	git cat-file blob "$2" | head -1 | grep "^#\!/" > /dev/null
 }
 mode_not_exe () {
 	echo "The file '$file' has looks executable but does not have an executable mode."