git-gui: Teach console widget to use git_read

git-gui: Teach console widget to use git_read

Now that we are pretty strict about setting up own absolute paths to
any git helper (saving a marginal runtime cost to resolve the tool)
we can do the same in our console widget by making sure all console
execs go through git_read if they are a git subcommand, and if not
make sure they at least try to use the Tcl 2>@1 IO redirection if
possible, as it should be faster than |& cat.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>