bst --track modifies bst files even when their source refs are unchanged

Summary

After discussion, it has been identified that:

• ruamel.yaml is imperfect, when we rewrite a file, some indentation will be changed and whitespace will change in some places
  • it's still not horrible, comments and order is properly preserved, and whitespace is preserved in most cases, but it could be better
• After introducing the project.refs feature (in commits referred to in the discussion below), it seems that we are rewriting the element .bst files at bst track time even when the ref has not changed.

Steps to reproduce

• bst --track an element with custom indentation
• git diff the file, and see indentation changes

What is the current bug behavior?

When running bst-track, indentation, ordering and comments are not preserved.

What is the expected correct behavior?

bst track should update the version/sha, but respect the files formatting.

Relevant logs and/or screenshots

freedesktop-sdk/freedesktop-sdk!318 (diffs)

Example element

kind: manual
description: Build curl
# Comment comment pls remember
depends:
- base.bst
# ruamel isn't quite working
sources:
- kind: git
  url: https://github.com/curl/curl.git

config:
  build-commands:
  - ./buildconf
  - ./configure
  - make

After bst track :

config:
  build-commands:
  - ./buildconf
  - ./configure
  - make
depends:
- base.bst
description: Build curl
kind: manual
sources:
- kind: git
  ref:
  url: https://github.com/curl/curl.git

Possible fixes

review changes in 19cad981 as this is where the regression appears to have been caused.

BuildStream version affected:

bst version: 1.1.3+4.g0916d81e