Allow branches to be specified when building TiddlyWiki5

[Jermolene]

To address https://github.com/Jermolene/TiddlyWiki5/issues/3623 we need to extend the build scripts so that they can build the prerelease with the master branch of TW5 while using the tiddlywiki-com branch when rebuilding the current release with the latest documentation updates.

A complexity with the current build scripts is that they originated as the scripts I (@Jermolene) was using for my everyday development of TiddlyWiki5, and they still serve that purpose alongside being used to prepare builds for tiddlywiki.com. That is why the current scripts reach into the filing system for the files they need, rather than pulling them down afresh from git. In preparation for introducing continuous integration, it is proposed to drop support for using these scripts for local development and instead focus on the scenarios we need for CI. This means that the scripts will be more self-contained, pulling everything they need from git.

Breakdown of tasks:

• [ ] Rationalise the existing build scripts -- there are a number of scripts designed for interactive use that duplicate fragments of bld.sh that can be removed
• [ ] Add new development build scripts to the TW5 repo
• [ ] Update production build scripts here in build.jermolene.github.io to pull down appropriate branch
• [ ] Update all build scripts to optionally include a tiddler with enough git status to allow the build to be tracked

[Jermolene]

@pmario - could you kindly point me at the CI scripts we were working on a couple of years ago? Many thanks.

[Jermolene]

This has been addressed in v5.1.20.