Only update changed collate

[Geoff99]

Hi @hadley

This pull request relates to devtools issue r-lib/devtools#723

@renozao neatened up my first quick fix to make update_collate rewrite the Collate entry in the DESCRIPTION file only when roxgyen2 finds changes generated by #' @includes in the package. He also wrote a unit test as well.

This code won't guarantee that devtools can run safely in parallel (that's a much bigger job I think) but I think it will resolve the use case which generated the devtools issue (or at least return control of the problem to anyone running multiple load_alls at the same time on a package)

Happy to respond to any changes you'd like made. Thanks in advance for looking over this.

[renozao]

I don't think we need to tackle the parallel loading in general.

I believe that for plain R packages it should be enough to ensure that no files is changed if not necessary. The use case is that the package has first been loaded by load_all in a separate session before running the parallel jobs, so that each job, when individually loading the package with load_all should not overwrite anything.

[hadley]

Could you please add a bullet point to NEWS?

[Geoff99]

Bullet point about update_collate only rewriting the Collate entry after changes has been added to NEWS, as requested.

[hadley]

Can you please put the NEWS under the top-most bullet point? (should be under 4.1.1.9000) (and similarly for your other PR)

[Geoff99]

NEWS entries moved.

[hadley]

Thanks!