git-gui: Use builtin version of 'git gc'.

git-gui: Use builtin version of 'git gc'.

Technically the new git-gc command is strictly Porcelain; its invoking
multiple plumbing commands to do its work.  Since git-gui tries to not
rely on Porclain we shouldn't be invoking git-gc directly, instead we
should perform its tasks on our own.

To make this easy I've created console_chain, which takes a list of
tasks to perform and runs them all in the same console window.  If
any individual task fails then the chain stops running and the window
shows a failure bar.  Only once all tasks have been completed will it
invoke console_done with a successful status.

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