cargo upgrade --skip-compatible rewrites Cargo.toml dependencies

[djc]

This is not what I expected to happen:

I would expect that cargo upgrade (a) leaves dependency specifications in the same format as it found them, (b) doesn't change them if the new version if semver-compatible while --skip-compatible is passed.

[epage]

Note: #444 is the more general request to not change the style ever

[epage]

This was resolved in #725