git-gui: pass redirections as separate argument to _open_stdout_stderr

git-gui: pass redirections as separate argument to _open_stdout_stderr

We are going to treat command arguments and redirections differently to
avoid passing arguments that look like redirections to the command
accidentally. To do so, it will be necessary to know which arguments
are intentional redirections. Rewrite direct callers of
_open_stdout_stderr to pass intentional redirections as a second
(optional) argument.

Passing arbitrary arguments is not safe right now, but we rename it
to safe_open_command anyway to avoid having to touch the call sites
again later when we make it actually safe.

We cannot make the function safe right away because one caller is
git_read, which does not yet know which of its arguments are
redirections. This is the topic of the next commit.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>