git-gui: pass redirections as separate argument to git_read

git-gui: pass redirections as separate argument to git_read

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 call sites of git_read
to pass intentional redirections as a second (optional) argument.

git_read defers to safe_open_command, but we cannot make it safe, yet,
because one of the callers of git_read is proc git, 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>