git-gc: improve wording of --auto notification

git-gc: improve wording of --auto notification

The previous message had too much of a "boy, you should
really turn off this annoying gc" flair to it. Instead,
let's make sure the user understands what is happening, that
they can run it themselves, and where to find more info.

Suggested by Brian Gernhardt.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>