Remove magic default value as per review comment

README.md

@@ -685,33 +685,32 @@ with Overcommit without writing any Ruby code in a similar way as
 
 These special line-aware command hooks behave and are configured the same way
 as the Git ones, except only file arguments get passed to them.
-Also to enable the feature, they must use at least one of the following options,
-so that, using the command output:
+Also to enable them and for optimal use, one must configure them as explained
+below, so that, using the command output:
 - differentiating between warnings and errors becomes possible
 - modified lines can be detected and acted upon as defined by
   the `problem_on_unmodified_line`, `requires_files`, `include` and `exclude`
   [hook options](#hook-options)
 
 **Warning**: Only the command's standard output stream is considered for now,
 *not* its standard error stream.
-If you do not need to change the default values for any other option,
-then the `extract_messages_from` option has to be specified.
-Its value does not matter for now, but it should be set to `stdout`
-to avoid problems in the future.
 
 To differentiate between warning and error messages,
 the `warning_message_type_pattern` option may be specified:
 the `type` field of the `message_pattern` regexp below must then include
 the `warning_message_type_pattern` option's text.
 
 The `message_pattern` option specifies the format of the command's messages.
-It is a optional [(Ruby) regexp][RubyRE], which if present must at least define
+It is mandatory, must be a [(Ruby) regexp][RubyRE], and must define at least
 a `file` [named capture group][RubyRENCG].
 The only other allowed ones are `line` and `type`, which when specified
 enable detection of modified lines and warnings respectively.
 
-**Note**: The default value for this option is often adequate:
-it generalizes the quasi-standard [GNU/Emacs-style error format][GNUEerrf],
+**Tip**: The following value for this option is often adequate:
+```ruby
+/^(?<file>(?:\w:)?[^:]+):(?<line>\d+):[^ ]* (?<type>[^ ]+)/
+```
+It generalizes the quasi-standard [GNU/Emacs-style error format][GNUEerrf],
 adding the most frequently used warning syntax to it.
 
 For example:

lib/overcommit/hook_loader/base.rb

@@ -22,14 +22,8 @@ def load_hooks
 
     private
 
-    # GNU/Emacs-style error format:
-    AD_HOC_HOOK_DEFAULT_MESSAGE_PATTERN =
-      /^(?<file>(?:\w:)?[^:]+):(?<line>\d+):[^ ]* (?<type>[^ ]+)/.freeze
-
     def is_hook_line_aware(hook_config)
-      hook_config['extract_messages_from'] ||
-        hook_config['message_pattern'] ||
-        hook_config['warning_message_type_pattern']
+      hook_config['message_pattern']
     end
 
     def create_line_aware_command_hook_class(hook_base) # rubocop:disable Metrics/MethodLength
@@ -53,8 +47,7 @@ def extract_messages(line_aware_config, result)
           warning_message_type_pattern = line_aware_config[:warning_message_type_pattern]
           Overcommit::Utils::MessagesUtils.extract_messages(
             result.stdout.split("\n"),
-            line_aware_config[:message_pattern] ||
-              AD_HOC_HOOK_DEFAULT_MESSAGE_PATTERN,
+            line_aware_config[:message_pattern],
             Overcommit::Utils::MessagesUtils.create_type_categorizer(
               warning_message_type_pattern
             )

spec/overcommit/hook_loader/plugin_hook_loader_spec.rb

@@ -78,17 +78,18 @@
             flags:
             - "--format=emacs"
             include: '**/*.foo'
-          FooLintDefault:
+          FooLintRecommendedPattern:
             enabled: true
             command: ["foo", "lint"]
+            message_pattern: !ruby/regexp /^(?<file>(?:\w:)?[^:]+):(?<line>\d+):[^ ]* (?<type>[^ ]+)/
             warning_message_type_pattern: warning
             flags:
             - "--format=emacs"
             include: '**/*.foo'
-          FooLintDefaultNoWarnings:
+          FooLintRecommendedPatternNoWarnings:
             enabled: true
             command: ["foo", "lint"]
-            extract_messages_from: stdout
+            message_pattern: !ruby/regexp /^(?<file>(?:\w:)?[^:]+):(?<line>\d+):[^ ]* (?<type>[^ ]+)/
             flags:
             - "--format=emacs"
             include: '**/*.foo'
@@ -143,8 +144,8 @@
       it { should fail_hook }
     end
 
-    describe '(using default pattern)' do
-      let(:hook_name) { 'FooLintDefault' }
+    describe '(using recommended pattern)' do
+      let(:hook_name) { 'FooLintRecommendedPattern' }
 
       context 'when command fails with some warning message' do
         let(:result) do
@@ -175,8 +176,8 @@
       end
     end
 
-    describe '(using defaults)' do
-      let(:hook_name) { 'FooLintDefaultNoWarnings' }
+    describe '(using recommended pattern w/o warnings)' do
+      let(:hook_name) { 'FooLintRecommendedPatternNoWarnings' }
 
       context 'when command fails with some messages' do
         let(:result) do