Get suite running smoothly again

[benmelz]

Am planning on proposing/implementing a new feature, but first I noticed that there are a number of failing specs on main, so I figured I would start with that.

As written, this includes four main adjustments, two of which might actually be bugfixes:

• one spec was hanging indefinitely for me locally on a Thread.join call - explicitly capturing the child threads and joining them directly fixed it (might be machine dependent?)
• when an encoding is specified, the rails schema up to date hook was passing arguments incorrectly (kwarg containing encoding was being interpreted as the length positional arg) (possible bug)
• many specs were attempting to create file submodules without the protocol.file.allow=always config option causing failure
• alias detection for amendment appeared to be broken, due to regexp incompatibilities between ruby/git (git wasn't properly parsing \s in the value matcher for git config --regexp [key] [value] command), so moved that matching into ruby (possible bug, also maybe machine/git build opt dependent)
• dropped ruby 2.6 from the test matrix - it appears that it's far enough EOL that github has trouble even setting it up. I can attempt to add it back/get it working, but I figured it might be time to ditch it

*I don't have a windows machine, so I'm not gonna be much help with those CI builds (should note fails appear unrelated to anything I've done here).

[sds]

Would be open to dropping support for Ruby 2.7, given it's also EoL (but if we have tests passing, fine to keep).