Fix JavaCheckstyle message processing regex

Error lines now start with `[ERROR]`, which was apparently not always
the case. This was causing errors like the following:

    No such file or directory @ rb_sysopen - [ERROR] /path/to/src/...

Fix it by expanding the regex to ignore an optional prefix in square
brackets.

Fixes sds#358.

Author: sds

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * Fix `AuthorName`/`AuthorEmail` pre-commit hooks to respect
   `GIT_AUTHOR_NAME`/`GIT_AUTHOR_EMAIL` environment variables, respectively
+* Fix `JavaCheckstyle` pre-commit hook to ignore `[ERROR]` prefix when parsing
+  output messages
 
 ## 0.32.0
 

lib/overcommit/hook/pre_commit/java_checkstyle.rb

@@ -3,7 +3,7 @@ module Overcommit::Hook::PreCommit
   #
   # @see http://checkstyle.sourceforge.net/
   class JavaCheckstyle < Base
-    MESSAGE_REGEX = /^(?<file>(?:\w:)?[^:]+):(?<line>\d+)/
+    MESSAGE_REGEX = /^(\[[^\]]+\]\s+)?(?<file>(?:\w:)?[^:]+):(?<line>\d+)/
 
     def run
       result = execute(command, args: applicable_files)