push: add an advice on unqualified <dst> push

Add an advice to the recently improved error message added in
f8aae12034 ("push: allow unqualified dest refspecs to DWIM",
2008-04-23).

Now with advice.pushUnqualifiedRefName=true (on by default) we show a
hint about how to proceed:

    $ ./git-push avar v2.19.0^{commit}:newbranch -n
    error: The destination you provided is not a full refname (i.e.,
    starting with "refs/"). We tried to guess what you meant by:

    - Looking for a ref that matches 'newbranch' on the remote side.
    - Checking if the <src> being pushed ('v2.19.0^{commit}')
      is a ref in "refs/{heads,tags}/". If so we add a corresponding
      refs/{heads,tags}/ prefix on the remote side.

    Neither worked, so we gave up. You must fully qualify the ref.
    hint: The <src> part of the refspec is a commit object.
    hint: Did you mean to create a new branch by pushing to
    hint: 'v2.19.0^{commit}:refs/heads/newbranch'?
    error: failed to push some refs to 'git@github.com:avar/git.git'

When trying to push a tag, tree or a blob we suggest that perhaps the
user meant to push them to refs/tags/ instead.

The if/else duplication for all of OBJ_{COMMIT,TAG,TREE,BLOB} is
unfortunate, but is required to correctly mark the messages for
translation. See the discussion in
<87r2gxebsi.fsf@evledraar.gmail.com> about that.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

advice.h

@@ -9,6 +9,7 @@ extern int advice_push_non_ff_matching;
 extern int advice_push_already_exists;
 extern int advice_push_fetch_first;
 extern int advice_push_needs_force;
+extern int advice_push_unqualified_ref_name;
 extern int advice_status_hints;
 extern int advice_status_u_option;
 extern int advice_commit_before_merge;