Fail early on invalid input (raise ... unless ...)

This intention of this change is to make the normal flow of execution
easier to read, and to prevent mistakes in deeply nested if-else trees.

lib/gitlab_shell.rb

@@ -18,7 +18,8 @@ class GitlabShell
     if @origin_cmd
       parse_cmd
 
-      if git_cmds.include?(@git_cmd)
+      raise DisallowedCommandError unless git_cmds.include?(@git_cmd)
+
       ENV['GL_ID'] = @key_id
 
       access = api.check_access(@git_cmd, @repo_name, @key_id, '_any')
@@ -30,9 +31,6 @@ class GitlabShell
         $logger.warn message
         puts access.message
       end
-      else
-        raise DisallowedCommandError
-      end
     else
       puts "Welcome to GitLab, #{username}!"
     end
@@ -51,14 +49,12 @@ class GitlabShell
     @git_cmd = args.first
 
     if @git_cmd == 'git-annex-shell'
-      if @config.git_annex_enabled?
+      raise DisallowedCommandError unless @config.git_annex_enabled?
+
       @repo_name = escape_path(args[2].sub(/\A\/~\//, ''))
 
       # Make sure repository has git-annex enabled
       init_git_annex(@repo_name)
-      else
-        raise DisallowedCommandError
-      end
     else
       raise DisallowedCommandError unless args.count == 2
       @repo_name = escape_path(args.last)
@@ -73,7 +69,8 @@ class GitlabShell
     repo_full_path = File.join(repos_path, repo_name)
 
     if @git_cmd == 'git-annex-shell'
-      if @config.git_annex_enabled?
+      raise DisallowedCommandError unless @config.git_annex_enabled?
+
       args = Shellwords.shellwords(@origin_cmd)
       parsed_args =
         args.map do |arg|
@@ -88,9 +85,6 @@ class GitlabShell
 
       $logger.info "gitlab-shell: executing git-annex command <#{parsed_args.join(' ')}> for #{log_username}."
       exec_cmd(*parsed_args)
-      else
-        raise DisallowedCommandError
-      end
     else
       $logger.info "gitlab-shell: executing git command <#{@git_cmd} #{repo_full_path}> for #{log_username}."
       exec_cmd(@git_cmd, repo_full_path)