Add "Diff selected with..." option to command palette

[themilkman]

Problem description

On reviews, I often need to find a specific commit to compare it with HEAD/ some other commit (or even branch). Therefore I select it. Now, I need to scroll to the target to compare in order to diff it. That's mostly annoying.

Preferred solution

Having a command palette entry "Diff selected with..." that will open a list with HEAD, tags, branches (to target their head) and commit hashes to select. Afterwards both are selected and the diff will be shown.

Alternatives
More like an addition than an alternative would be another CP entry with "Show Diff between..." which will prompt twice for both, source and target with the same elements as proposed before.

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Edit Okay, this issue is quite similar to #321 and #255