all: os/exec => internal/xec

[abhinav]

Switch all uses of "os/exec" to "internal/xec"
to centralize all external command execution.

Besides making testing easier,
this allows for shipping functionality like
"all subcommands will get this environment variable set"
(which is something I'm planning on adding).

• Vet all call sites again to make sure that nothing that was previously surfacing stderr is now consuming it.

[skip changelog]: no user facing changes

[abhinav]

This change is part of the following stack:

• refac: extract gitCmd into re-usable xec.Cmd #986
  • all: os/exec => internal/xec #988 ◀

_{Change managed by git-spice.}