Store and display custom Git branch name in version information

[Calinou]

This information is visible in the project manager, editor, command line help and About dialog (trimmed to 20 characters in places where it's displayed in the editor). It can also be queried using Engine.get_version_info()["branch"].

This helps avoid mistakes when testing multiple builds from various branches.

Internally, the constant is VERSION_GIT_BRANCH as VERSION_BRANCH is already used to store the major.minor version number.

If the branch name is a standard Godot maintenance branch (such as master or 4.1), the branch name is an empty string (or unknown in Engine.get_version_info()'s return value).

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