Reword placeholder input text during base branch selection step

[justinrobots]

We've noticed some confusion at the step where users need to select a base branch to create a new branch from. They usually start typing the name of the branch instead of selecting a base, which removes all quick pick options to proceed because no branches match the input.

We should consider updating text to be more clear (see image for current text).

• Title bar: "Select base for new branch"
• Input placeholder: "Select base to create the new branch from (or enter a revision using #)".

[axosoft-ramint]

Done in commit: 3b4e917

[github-actions]

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