Need to trap stderr separately from stdout

[bobbingwide]

In includes/class-git.php the command method currently redirects stderr to stdout. This can cause problems downstream, where the output is misinterpreted as files to process, for example. It would be better if this could be returned and handled separately.

See https://stackoverflow.com/questions/2320608/php-stderr-after-exec for one option

[bobbingwide]

When performing an update of a plugin for wp-a2z.org e.g. update jetpack 8.1 the solution using execute_with_proc_open hangs, or appears to hang during 'git add .'

Don't know why.

Workaround:

revert to using the execute method.