When running `bundle lock --update <name>`, checkout locked revision of unrelated git sources directly

Since Bundler 2.4, we will try to checkout any branch specified in the
Gemfile, while until Bundler 2.3 we would directly checkout the locked
revision.

This should not make any difference in most situations, but in some edge
cases, like if the branch specified in the `Gemfile` has been renamed,
but the locked revision still exist, it causes an error now while before
it would update the lockfile without issues.

I debated which behavior was best, since I was not sure. But my
conclusion is that if the situation does not require expiring the
lockfile source in favor of the Gemfile source, we should use the locked
revision directly and proceed happily. So I restored Bundler 2.3
behavior.

I think this is consistent with how yanked gems are handled, for example.

Of course, if explicitly updating the git source itself, or all gems, we
will still get any errors like missing branches related to the git source.

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