counsel.el: Call du process directly

[basil-conto]

(counsel--git-grep-count-func-default): Call git and du directly to avoid shell incompatibilities in redirection.

Cc: @articuluxe, @icarus-sparry, @mikecrowe, @shalin24
Re: #1470, #1558
Fixes #1502

P.S. Sorry for not commenting on this at the time, but why do counsel--git-grep-count, counsel--git-grep-count-func, and counsel--git-grep-count-func-default have double hyphens in their names, if they're intended as a means for (advanced) user customisation?

Re: #1470 (comment)
Wouldn't it be cleaner to allow values of ivy-more-chars-alist to be functions, so that ivy-more-chars is no longer guarded ad-hoc by (> counsel--git-grep-count counsel--git-grep-count-threshold) in counsel-git-grep-function?

[basil-conto]

As was discussed in #1470, #1502, and #1558, it would be nice to create a uniform, flexible, and human-readable wrapper around process invocation, but I think it's more pressing to solve the current issue first and worry about refactoring later.

[abo-abo]

Thanks. Please also have a look at the new wrapper. I used call-process, but maybe we should use process-file.

[basil-conto]

Please also have a look at the new wrapper.

Thanks, I've submitted a few suggestions in #1828.

I used call-process, but maybe we should use process-file.

I think so too, given shell commands obey file handlers.