git-gui: remove option --stderr from git_read

git-gui: remove option --stderr from git_read

Some callers of git_read want to redirect stderr of the invoked command
to stdout.  The function offers option --stderr for this purpose.
However, the option only appends 2>@1 to the commands.  The callers can
do that themselves. In lib/console.tcl we even have a caller that
already knew implictly what --stderr does behind the scenes.

This is a preparation for a later change where we want to make git_read
non-variadic. Then it cannot have optional leading arguments.

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