append option for user-config command

[gbaz]

• Patches conform to the [coding conventions (https://github.com/haskell/cabal/#conventions).
• Any changes that could be relevant to users have been recorded in the changelog.

Should resolve #4901

This adds an additional option for user-config, as --append= or -a. This can be used multiple times, each time adding a new line to the delta on user-config files for init update or diff. This lets tools like the platform installer pass in, e.g. msys directories for extra-prog-path and extra-lib-dirs etc.

Tested on some example files, and new-test passes.

[gbaz]

@randen once this is merged we can update the windows installer to call either cabal init or cabal update with the relevant msys lines (I guess since we don't have an init-or-update it'll need to be a two step process to check for the existence of the file first...)

[gbaz]

(travis error seems spurious)

[23Skidoo]

So, if we're updating an already existing config that has an extra-prog-path entry, it will have two such entries. Does cabal-install handle this case correctly?

[23Skidoo]

Looks OK. Is --append hidden from the help output? Or do you think that it should be exposed to users?

[23Skidoo]

I'd call this --append-lines. Note that you'll still be able to say --append=foo, since --append is an unique prefix.

[gbaz]

So, if we're updating an already existing config that has an extra-prog-path entry, it will have two such entries. Does cabal-install handle this case correctly?

Actually, it won't have two entries. The semantics of update in general will replace the field, not augment it. That said, cabal-install does handle two such lines correctly.

It may be the "right thing" to augment some lines instead of replace them with update semantics, but we'd have to handle this case by case for all config lines, and that seems too broad for this PR.

[gbaz]

As such it might be clearer to name the setting "augment" and change the doc to read "Additional setting to augment the config file (replacing a previous setting if it existed)." It isn't hidden from the help output -- no reason not to expose this to end users, imho.

If you agree on the name I'll update the PR.

[23Skidoo]

Sorry for not reacting earlier, was travelling for work. I haven't managed to come up with a better name, so let's use "augment".