cherry-pick/revert: Use advise() for hints

cherry-pick/revert: Use advise() for hints

When cherry-pick fails after picking a large series of commits, it can
be hard to pick out the error message and advice.  Prefix the advice
with “hint: ” to help.

Before:

    error: could not apply 7ab78c9... foo
      After resolving the conflicts,
    mark the corrected paths with 'git add <paths>' or 'git rm <paths>'
    and commit the result with:

            git commit -c 7ab78c9a7898b87127365478431289cb98f8d98f

After:

    error: could not apply 7ab78c9... foo
    hint: after resolving the conflicts, mark the corrected paths
    hint: with 'git add <paths>' or 'git rm <paths>'
    hint: and commit the result with 'git commit -c 7ab78c9'

Noticed-by: Thomas Rast <trast@student.ethz.ch>
Encouraged-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>