pr-head and pr-merge break if a tag is updated

[gilles-peskine-arm]

The pr-head and pr-merge jobs completely break if a tag is updated in the repository. This isn't something we do in normal operation, but we may want to do it occasionally for tags that aren't meant to be consumed by users, or we may do it occasionally as a mistake when pushing the wrong thing.

Example (https://jenkins-mbedtls.oss.arm.com/job/mbed-tls-pr-head/job/PR-6720-head/4/console):

Pull request #6720 updated
02:52:17 Connecting to https://api.github.com/ using oss-mbed-tls-bot/****** (oss-mbed-tls-bot Github API Authentication credentials)
Loading trusted files from base branch development at 262851df1cc4f090b3e2920cd647d5e22c9bb2f1 rather than 70c1f7be4c2490c4d3bff87b7060dc5a6735a538

GitHub has been notified of this commit’s build result

hudson.plugins.git.GitException: Command "git fetch --tags --progress origin +refs/heads/*:refs/remotes/origin/* --prune" returned status code 1:
stdout: 
stderr: From https://github.com/Mbed-TLS/mbedtls
 ! [rejected]            features/new-code-style/test/new/6863 -> features/new-code-style/test/new/6863  (would clobber existing tag)

and the job stops there.

I don't know if we can actually fix that, or if it's a brittleness in the multibranch pipeline plugin.