fix up amend alias detection

lib/overcommit/hook_context/helpers/file_modifications.rb

@@ -12,7 +12,7 @@ def amendment?
         cmd = Overcommit::Utils.parent_command
         return unless cmd
 
-        amend_pattern = 'commit(\s.*)?\s--amend(\s|$)'
+        amend_pattern = /commit(\s.*)?\s--amend/
 
         # Since the ps command can return invalid byte sequences for commands
         # containing unicode characters, we replace the offending characters,
@@ -24,18 +24,11 @@ def amendment?
             encode('UTF-8')
         end
 
-        return @amendment if
-          # True if the command is a commit with the --amend flag
-          @amendment = !(/\s#{amend_pattern}/ =~ cmd).nil?
+        # True if the command is a commit with the --amend flag
+        return @amendment if @amendment = cmd.match?(amend_pattern)
 
         # Check for git aliases that call `commit --amend`
-        `git config --get-regexp "^alias\\." "#{amend_pattern}"`.
-          scan(/alias\.([-\w]+)/). # Extract the alias
-          each do |match|
-            return @amendment if
-              # True if the command uses a git alias for `commit --amend`
-              @amendment = !(/git(\.exe)?\s+#{match[0]}/ =~ cmd).nil?
-          end
+        return @amendment if @amendment = command_is_amend_alias?(cmd, amend_pattern)
 
         @amendment
       end
@@ -74,6 +67,22 @@ def modified_lines_in_file(file)
         end
         @modified_lines[file]
       end
+
+      private
+
+      def command_is_amend_alias?(cmd, amend_pattern)
+        `git config --get-regexp "^alias"`.split("\n").each do |alias_def|
+          alias_map = alias_def.match /alias\.(?<to>[-\w]+)\s+(?<from>.+)/
+          next unless alias_map
+
+          alias_from_match = alias_map[:from].match? amend_pattern
+          alias_to_match = cmd.match? /git(\.exe)?\s+#{alias_map[:to]}/
+
+          # True if the command uses a git alias for `commit --amend`
+          return true if @amendment = alias_from_match && alias_to_match
+        end
+        false
+      end
     end
   end
 end