feat: allow to customize initial name on branch rename

[achmutov]

Currently, when renaming a branch, the input is prefilled with initial_branch_name, introduced in #1517.

However, at some point in time, Neogit used the current branch name. This feature was introduced in #904, but was lost over the time.

This PR introduces initial_branch_rename config feature. By default, the value is nil, which indicates current branch name. Otherwise, i.e. if string is provided, the value behaves like initial_branch_name.

Attaching a demo with:

• initial_branch_name set to initial_branch_name
• initial_branch_rename set to nil (default)

demo.mp4