Improve trace logging for pulls and processes (#22633) (#22812)

Improve trace logging for pulls and processes (#22633) (#22812)

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>