Simplify whitespace-linter script

Pegleg currently uses `find` to search files in the whitespace-linter
script. A more simplified approach could be taken by using `git grep`
instead. This method levarages .gitignore so a separate list of files
in the script no longer needs to be maintained. This is the method used
by Airship Armada.

Change-Id: I26a2a95f533b9ff62de784d004f25ade552a5b31

tools/gate/whitespace-linter.sh

@@ -1,16 +1,9 @@
 #!/usr/bin/env bash
 
-set -xe
-RES=$(find . \
-  -not -path "*/\.*" \
-  -not -path "*/doc/build/*" \
-  -not -path "*/doc/source/images/*" \
-  -not -path "*/htmlcov/*" \
-  -not -name "*.tgz" \
-  -not -name "*.pyc" \
-  -not -name "*.html" \
-  -type f -exec egrep -l " +$" {} \;)
+set -x
+
+RES=$(git grep -E -l " +$")
 
 if [[ -n $RES ]]; then
-  exit 1;
+  exit 1
 fi