Expands d{number} to be dirname $e{number}

[ghthor]

Considering the current plan for maintenance is going to be heavily focused on refactoring I'd rather not merge this feature addition at this time. But I'm curious about the use case that guided you to this feature, as it may be worth revisiting this in the future.

[JoeEnnever]

Ok, I'd be happy to re-submit this when the refactor is done.

I wrote this because I use pants as a build tool, where you end up operating on changed directories quite often. For example, if I have a changed file src/foo/Foo.scala, and want to recompile it with pants, I'd run

$ ./pants compile src/foo

With this change, if $e1 is set to Foo.scala, that becomes

$ exec_scmb_expand_args ./pants compile d1