Fix grep-based hooks to work with long file lists

Now that the `execute` helper supports passing in a separate list of
splittable arguments, we can use that version to avoid issues with long
lists of files exceeding the maximum command length for the operating
system.

Author: sds

lib/overcommit/hook/pre_commit/hard_tabs.rb

@@ -2,7 +2,7 @@ module Overcommit::Hook::PreCommit
   # Checks for hard tabs in files.
   class HardTabs < Base
     def run
-      result = execute(command + applicable_files)
+      result = execute(command, applicable_files)
 
       extract_messages(
         result.stdout.split("\n"),

lib/overcommit/hook/pre_commit/local_paths_in_gemfile.rb

@@ -2,7 +2,7 @@ module Overcommit::Hook::PreCommit
   # Checks for local paths in files and issues a warning
   class LocalPathsInGemfile < Base
     def run
-      result = execute(command + applicable_files)
+      result = execute(command, applicable_files)
 
       unless result.stdout.empty?
         return :warn, "Avoid pointing to local paths in Gemfiles:\n#{result.stdout}"

lib/overcommit/hook/pre_commit/merge_conflicts.rb

@@ -2,7 +2,7 @@ module Overcommit::Hook::PreCommit
   # Checks for unresolved merge conflicts
   class MergeConflicts < Base
     def run
-      result = execute(command + applicable_files)
+      result = execute(command, applicable_files)
 
       unless result.stdout.empty?
         return :fail, "Merge conflict markers detected:\n#{result.stdout}"

lib/overcommit/hook/pre_commit/trailing_whitespace.rb

@@ -2,7 +2,7 @@ module Overcommit::Hook::PreCommit
   # Checks for trailing whitespace in files.
   class TrailingWhitespace < Base
     def run
-      result = execute(command + applicable_files)
+      result = execute(command, applicable_files)
 
       extract_messages(
         result.stdout.split("\n"),