Force specific hash when building a single docs branch

[philwebb]

Currently when a docs build is triggered for a branch, the latest version of that branch will always be used. This can cause problem if a subsequent build adds meta-data and hasn't completed in time.

This happened with commit f5f888d which triggered a docs build that actually used afcc780.

If we pass the sha in when we trigger the build, we should be able to git reset the branch before starting Antora.