git-gui: break out a separate function git_read_nice

git-gui: break out a separate function git_read_nice

There are two callers of git_read that request special treatment using
option --nice. Rewrite them to call a new function git_read_nice that
does the special treatment. Now we can remove all option treatment from
git_read.

git_write has the same capability, but there are no callers that
request --nice. Remove the feature without substitution.

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

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