Contributing to Ruby on Rails Guide: standardize git remote names [ci skip].

[jgorman]

Summary

As a new contributor, I found that the git commands in the Contributing to Ruby on Rails
Guide are confusing.

The git remote name for rails/rails is "upstream", "rails", and "origin" in different parts of the guide.

The git remote name for the forked repository on the user's github account is "mine" and "origin". Sometimes "origin" refers to rails/rails and sometimes it refers to username/rails!

Also, the contribution branch name is "my_new_branch" and "my_pull_request" which is confusing.

This pull request standardizes the remote and branch names and removes some unhelpful instructions. "rails" now refers to rails/rails and "fork" now refers to username/rails. The contribution branch name is now "my_new_branch" throughout.

I have tested the commands.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @rafaelfranca (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

This repository is being automatically checked for code quality issues using Code Climate. You can see results for this analysis in the PR status below. Newly introduced issues should be fixed before a Pull Request is considered ready to review.

Please see the contribution instructions for more information.

[bogdanvlviv]

Thanks!