Linemapped command output indents the trailing newline

[schneems]

This code:

    const CMD_INDENT: &'static str = "      ";
//...
            let result = command
                .output_and_write_streams(
                    line_mapped(std::io::stdout(), add_prefix(CMD_INDENT)),
                    line_mapped(std::io::stderr(), add_prefix(CMD_INDENT)),
                )

Expected: That it only affects the outputlines of the command being run.
Actual: It adds indentation to the trailing newline which affects the next output.

Example:

- Bundle install
  - Running `BUNDLE_BIN="/layers/heroku_ruby/gems/bin" BUNDLE_CLEAN="1" BUNDLE_DEPLOYMENT="1" BUNDLE_GEMFILE="/workspace/Gemfile" BUNDLE_PATH="/layers/heroku_ruby/gems" BUNDLE_WITHOUT="development:test" bundle install`
      Fetching gem metadata from https://rubygems.org/..
      Fetching rake 13.0.6
      Installing rake 13.0.6
      Fetching rack 2.2.3
      Fetching webrick 1.7.0
      Using bundler 2.4.5
      Installing webrick 1.7.0
      Installing rack 2.2.3
      Bundle complete! 3 Gemfile dependencies, 4 gems now installed.
      Gems in the groups 'development' and 'test' were not installed.
      Bundled gems are installed into `/layers/heroku_ruby/gems`
              - Done (5.733s)

The bundle install command returns a newline as the very last character and the line_mapped prefix of is added to it and then the next line is appended with additional indentation.

The short term workaround is to add a println!() afterwards so it looks like this:

- Bundle install
  - Running `BUNDLE_BIN="/layers/heroku_ruby/gems/bin" BUNDLE_CLEAN="1" BUNDLE_DEPLOYMENT="1" BUNDLE_GEMFILE="/workspace/Gemfile" BUNDLE_PATH="/layers/heroku_ruby/gems" BUNDLE_WITHOUT="development:test" bundle install`
      Fetching gem metadata from https://rubygems.org/..
      Fetching rake 13.0.6
      Installing rake 13.0.6
      Fetching rack 2.2.3
      Fetching webrick 1.7.0
      Using bundler 2.4.5
      Installing webrick 1.7.0
      Installing rack 2.2.3
      Bundle complete! 3 Gemfile dependencies, 4 gems now installed.
      Gems in the groups 'development' and 'test' were not installed.
      Bundled gems are installed into `/layers/heroku_ruby/gems`

  - Done (5.733s)

However long term it would be nice to: not indent the final trailing newline or introduce a new method with that behavior

[edmorley]

@schneems Is this issue fixed by the new build_output module? Or will it need a separate fix? Do we need any new tests in this repo for it, or is it covered by the tests added in #721?