Support "blame following" in when git blaming

[olejorgenb]

In vscode / GitLens this is called "open blame prior to this change"

Jetbrains: https://www.jetbrains.com/help/pycharm/investigate-changes.html#annotate_previous_revision

You activate this for a specific line and the editor show a read only view of the file at that revision with blame still activated.

Perhaps this will come more or less for free once "open file at revision" is implemented though? (#16827 is kinda about that)

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This is an feature I occasionally miss enough to fire up vscode/jetbrains. Very useful to quickly understand the history of a code section.

[bjorn]

I also miss this feature many times. It is available in Qt Creator as "Blame Parent Revision" when right-clicking the commit hash in the blame side bar:

It is also available in git gui blame ... as "Blame Parent Commit":

Quite often the blamed code was later reformatted or affected by refactoring and then you need this feature to navigate to the original introduction of the code.

[bjorn]

It is also available in the GitHub blame view:

(though I'm personally not a fan of the clutter caused by this repeated button, which may just have been to avoid overriding the context menu)