Backport #22633
Our trace logging is far from perfect and is difficult to follow.
This PR:
* Add trace logging for process manager add and remove.
* Fixes an errant read file for git refs in getMergeCommit
* Brings in the pullrequest `String` and `ColorFormat` methods
introduced in #22568
* Adds a lot more logging in to testPR etc.
Ref #22578
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Signed-off-by: Andrew Thornton <art27@cantab.net>