reset: make the reminder output consistent with "checkout"

reset: make the reminder output consistent with "checkout"

git reset without argument displays a summary of the local modification,
like this:

    $ git reset
    Makefile: locally modified

Some people have problems with this; they look like an error message.

This patch makes its output mimic how "git checkout $another_branch"
reports the paths with local modifications.  "git add --refresh --verbose"
is changed in the same way.

It also adds a header to make it clear that the output is informative,
and not an error.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>