Add SelectNextForkPointAsDiffBase hotkey

[mstv]

Proposed changes

Feature: Quickly view the changes made in a branch.

• Add SelectNextForkPointAsDiffBase hotkey Ctrl+K
  Searches for the next fork point and selects it in addition to the single-selected revision.
  If two (or more) revisions are selected, the last selection is replaced with its next fork point.
  A fork point is a parent revision with multiple children or the head of any branch.
• Expose the existing command GoToMergeBase to HotkeySettingsManager and assign Ctrl+Shift+K

Hint: Hit Ctrl+K, then Ctrl+Shift+K in order to select the fork point only.

Screenshots

After

initial	after Ctrl+K

Test methodology

• manual

Test environment(s)

• Git Extensions 33.33.33
• Build 69c7bc6
• Git 2.35.1.windows.1
• Microsoft Windows NT 10.0.19044.0
• .NET 5.0.12
• DPI 96dpi (no scaling)

Merge strategy

I agree that the maintainer squash merge this PR (if the commit message is clear).

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✒️ I contribute this code under The Developer Certificate of Origin.

[mstv]

I am going to merge this in one day.