Add an option to explain commits from all changes on a branch

[justinrobots]

Description

There should be an option to explain commits for all changes on a branch that is accessible from multiple places:

• in the context menu when right-clicking a branch ref in the graph
• as a command palette command
• from branches in other GitLens views

Add the command:

• "Explain Unpushed Changed"
• Modify "Explain Branch Changes" - by trying to find user-configured merge base.

Follow-ups

Originally it was a ticket for "Recompose", but then "Explain" appeared from nowhere and eventually, "Recompose" has been moved out to wait for readiness of the CommitComposer. The follow up PR for recompose is here: #4644 the original description of the current ticket has been moved there. Probably, an issue needs to be created as well.

Verification steps

• test how each new menu item works
• test that new menu items are visible when and only when there are commits (unpushed or unmerged): both in graph and in Branch View.
• test how "Explain Branch Changes" works because it's base branch has been modified in getBranchMergeTargetName
• test how other features that depend on getBranchMergeTargetName
  • Get Branch Contributions Overview
  • Comparison default

[axosoft-ramint]

Where are the newly generated commits committed to? Is it onto a new branch, from the same starting point as the previous one? Or are the commits of the target branch completely overwritten by the new ones?

[justinrobots]

Where are the newly generated commits committed to? Is it onto a new branch, from the same starting point as the previous one? Or are the commits of the target branch completely overwritten by the new ones?

@axosoft-ramint The output of the re-compose function today is creating a new branch, correct? I assumed we would follow that same pattern and re-compose all commits from the selected branch into a new branch.