[CLI] Don't hardcode remote name

[tidoust]

When the --pr option is set, the CLI command needs to push committed changes to GitHub before it can create a pull request. The code currently assumes that the name of the remote to use is origin. That may not be the case, especially if the code is forked (or if someone who's like me does not like the name origin and decides to rename it). The code should rather look at possible remotes and choose the right one (essentially, the one that targets w3c/browser-specs on GitHub).