[EWS] clean-up-git-repo step tries to delete invalid branches

[David Kilzer (:ddkilzer)]

[EWS] clean-up-git-repo step tries to delete invalid branches:

```
/bin/sh -c 'git branch | grep -v '"'"' main$'"'"' | xargs git branch -D || true'
 in dir /Volumes/Data/worker/iOS-16-Build-EWS/build (timeout 300 secs)
 watching logfiles {}
 argv: [b'/bin/sh', b'-c', b"git branch | grep -v ' main$' | xargs git branch -D || true"]
 using PTY: False
error: branch '*' not found.
error: branch '(HEAD' not found.
error: branch 'detached' not found.
error: branch 'at' not found.
error: branch 'origin/main)' not found.
Deleted branch fix171805-svg-textLength (was 78a3a1afebe9).
program finished with exit code 0
elapsedTime=0.099196
```

Example:  <https://ews-build.webkit.org/#/builders/77/builds/12461>

[David Kilzer (:ddkilzer)]

Maybe the step should do something like `git checkout main` first before running this command?

[Radar WebKit Bug Importer]

<rdar://problem/104512536>

[David Kilzer (:ddkilzer)]

Pull request: https://github.com/WebKit/WebKit/pull/16877

[EWS]

Committed 267090@main (318e60ffceea): <https://commits.webkit.org/267090@main>

Reviewed commits have been landed. Closing PR #16877 and removing active labels.