Fix TrailingWhitespace to use extract_messages

Previously, if `problem_on_unmodified_line` was set to something other
than `report`, this hook would still report an error for lines the user
didn't modify.

Correct this by changing the hook to use `extract_messages` so line
information is included.

Author: sds

CHANGELOG.md

@@ -11,6 +11,9 @@
   [Scalariform](https://mdr.github.io/scalariform/)
 * Add `SlimLint` pre-commit hook that analyzes Slim templates with
   [Slim-Lint](https://github.com/sds/slim-lint)
+* Enhance `TrailingWhitespace` pre-commit hook to including line information in
+  errors, making it work as expected when `problem_on_unmodified_line` is set
+  to something other than `report`
 
 ## 0.24.0
 

lib/overcommit/hook/pre_commit/trailing_whitespace.rb

@@ -3,11 +3,11 @@ module Overcommit::Hook::PreCommit
   class TrailingWhitespace < Base
     def run
       result = execute(command + applicable_files)
-      unless result.stdout.empty?
-        return :fail, "Trailing whitespace detected:\n#{result.stdout}"
-      end
 
-      :pass
+      extract_messages(
+        result.stdout.split("\n"),
+        /^(?<file>[^:]+):(?<line>\d+)/,
+      )
     end
   end
 end