tools: update tools/codelines to use "git ls-files"

This generates a more accurate code count because 'make distclean'
doesn't always remove build files.

Author: idea from David Rowley

Discussion: https://postgr.es/m/aR4hoOotVHB7TXo5@momjian.us

Backpatch-through: master

src/tools/codelines

@@ -3,5 +3,5 @@
 # src/tools/codelines
 
 # This script is used to compute the total number of "C" lines in the release
-# This should be run from the top of the Git tree after a 'make distclean'
-find . -name '*.[chyl]' | xargs cat| wc -l
+# This should be run from the top of the Git tree.
+git ls-files -- '*.[chyl]' | xargs cat | wc -l