Add --update-only flag to cargo-add

[bjgill]

This adds the specified dependency/ies only if they are already present. Thus, it acts like a 'cargo upgrade' (see #74).

This PR is a partial solution to the problem of updating dependencies in Cargo.toml for crates in a workspace that do not share the same set of dependencies. Once this has been merged, one will then be able to upgrade the dependency foo for all crates by running cargo multi add --update-only foo (using cargo-multi).

[killercup]

Very nice PR!

Can you add a small integration test (or two) to assert that doing this on the CLI also works? Just append it to tests/cargo-add.rs :)

I'm not sure I really like the semantics of calling cargo add without adding something, but on the other hand I don't think cargo update will want to change the Cargo.toml. So, let's go for it.

[bjgill]

I've added a couple of tests for the mainline function.

Yes - I'm just going for something quick that reduces pain for a relatively common use-case. I suspect the next step would be to solve #74 properly by creating a new subcommand that took the output from cargo list and fed it into cargo add.

[killercup]

FYI, my current plans for this repo are:

• get a nice toml writer lib (open issue since 2015…)
• refactor cargo-{add,rm} to use state of the art deps -- or at least the same cargo uses
• make this useful as a library

Especially the last point would allow you to write a cargo upgrade relatively easily.

But for now:

@homu r+ okay, that service seems to finally be offline, i'll merge manually