git-gui: convert git_read*, git_write to be non-variadic

git-gui: convert git_read*, git_write to be non-variadic

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. As a preparation, convert git_read,
git_read_nice, and git_write to take just a single argument that is
the command in a list. Adjust all call sites accordingly.

In the future, this argument will be the regular command arguments and
a second argument will be the redirection operations.

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