6329: Stop printing “failed to update” message when changing sources r=segiddins a=segiddins

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

The problem was `bundle update` shows incorrect message when updating git gems

Closes #6325 

### What was your diagnosis of the problem?

My diagnosis was we need to check if the source has changed

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

My fix only prints the message when the source has not changed

Note that is does not yet fix the "updated the git sha" case, since the locked spec and the new spec actually share the same source object!

Co-authored-by: Samuel Giddins <segiddins@segiddins.me>
Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7127: Add git and branch options to `bundle add` r=deivid-rodriguez a=tokachev

Closes #6841 These changes will allow to add a gem with the git or branch options.

```
bundle add redis --git "https://github.com/redis/redis-rb" --branch "staging"
```

The result in Gemfile will be: 
```
gem "redis", "~> 4.0", :git => "https://github.com/redis/redis-rb", :branch => "staging"
```

Co-authored-by: Baumgarten <baumgarten@localhost.localdomain>

7128: Backport to workaround from ruby core. r=deivid-rodriguez a=hsbt

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

The current master branch couldn't invoke with the ruby core repository.

### What was your diagnosis of the problem?

1. We need to add explicitly declare `rspec` in spec_helper.rb
2. Some examples were failed on ruby core repository.

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

1. simply added.
2. update the `ruby_repo` labels for skipping to run.


Co-authored-by: SHIBATA Hiroshi <hsbt@ruby-lang.org>

6730: Print errors to stderr by default, and remove configuration option r=greysteil a=greysteil

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

The problem was #6729 - Bundler unexpectedly outputs error and warning messages to STDOUT.

### What was your diagnosis of the problem?

My diagnosis was that whilst very minorly breaking, this is essentially a bug fix, and should be considered for inclusion for Bundler 2.0 even if very few other breaking changes are.

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

My fix was so switch output for warning and error messages to STDERR, and remove the configuration option (as is redundant once the setup is flipped - anyone wanting to redirect those message to STDOUT could do so in their shell).

### Why did you choose this fix out of the possible options?

I chose this fix because I think the new behaviour is what everyone would expect, and we should get it out from behind its feature switch sooner rather than later.

Alternatively, we might want to keep the Bundler 2.0 release "purer" by only dropping Ruby versions in it - that's totally fine too, but I figured we should have the code to discus #6729, rather than doing it in abstract.

Co-authored-by: Grey Baker <greysteil@gmail.com>
Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>