Set default branch from "master" to "HEAD"

[MyKo101]

This PR replaces "master" within reference to repo branches with "HEAD", which grabs the latest commit from the default branch. This allows for repos that do not use the "master" convention to be targeted for download without the need to specify a reference (e.g. "main").

This will not effect repos that already use "master" as their default branch, as it will still be pulled. The only situation negatively effected by this PR is when a user is trying to target the "master" branch (and is thus using the previous default, ref="master") and it is not the default, which will be a rare edgecase.

It is also suggested that the remotes repository changes it's default branch to "main". Currently codecov requires a named branch and is incompatible with "HEAD", and so the badge (as it stands in this PR) will not display until this is done.

This resolves Issue #508

[jimhester]

You need to keep ?branch=xyz here and for the travis case, otherwise the badge would show the status of the last running branch instead of the default one.

[jimhester]

While I agree that a change from 'master' as the default branch of remotes would be preferable, I think we should wait to change the default branches on this and other repos until GitHub or git officially changes to a different default branch name. Right now the only 'official' word on this from GitHub is a twitter reply (https://twitter.com/natfriedman/status/1271253144442253312), and this change will likely break a number of workflows, so it needs to be done carefully.

For now could you update this PR to keep the default branch for remotes as 'master', and add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[MyKo101]

I understand the apprehension, as discussed on the Issue, some people's workflows will be largely affected. However, I believe it useful to have this ready to go and at some point this change will be needed.

I've added to the NEWS.md file as requested. This is actually my first PR (other than in my own repos), so any feedback on my style, etc... would be greatly appreciated.

[MyKo101]

I've addressed the comments by @gaborcsardi and @jimhester in my local clone of remotes, but wanted to fix a few other things before pushing it.

I currently have two errors being hit by the testthat.R section of devtools::check():

• base curl download redirects (@test-download.R#298)
  I believe that the url used here: http://httpbin.org/absolute-redirect/1 is broken. Suggestions to replace would be useful.
• install-github.R script is up to date (@test-script.R#10)
  I'm unfamiliar with make and brew, so I'm finding it difficult to repair this. I have tried running make in terminal and brew() in the same way as it is run in the test, but it is still failing. It's also worth mentioning that when I run devtools::test(), it is passing. And in my previous commit, this worked (both locally and with appveyor/travis). It would be useful if someone could point me in the right direction (I am eager to learn, however I would also like a quick-fix for this and I feel like it is probably a really simple solution that I don't know how to implement)

There is also a warning being flagged when I run devtools::test(), but I'm not sure how important this is:

test-install-github.R:208: warning: github_pull
'tar.exe -xf "C:\Users\mbrxsmbc\AppData\Local\Temp\RtmpMFii7R\file3a587a373233.tar.gz" -C 
"C:/Users/mbrxsmbc/AppData/Local/Temp/RtmpMFii7R/remotes3a5854ae6762"' returned error code 1

I have removed a reference to "master" in this test, however this warning remains with or without "master" included.

[MyKo101]

Currently the only reason for the build failures is due to an issue with postmanlabs/httpbin#617.

[jimhester]