Fix git source conservativeness

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

Git sources are not always conservative.

If you edit an attribute of a git source in the Gemfile (like its tag), but don't change the source itself (it's URL), then Bundler should behave consistently for dependencies of that source: it should re-resolve direct dependencies to satisfy the new change git source, but keep transitive dependencies already in the lockfile as much as possible.

What is your fix for the problem, implemented in this PR?

The fix is to allow Bundler to make the distinction between:

• A git source was completely changed to a different source. We want full unlock here.
• A git source is still the same, but the Gemfile is now pointing to a different reference. We want conservative subdependencies here.

This distinction is implemented by 9a0e0df. The other commits are just minor refactorings while trying to figure things out.

Fixes #6816.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags