Change incorrect "remote branch" to "remote tracking branch" in C code

Change incorrect "remote branch" to "remote tracking branch" in C code

(Just like we did for documentation already)

In the process, we change "non-remote branch" to "branch outside the
refs/remotes/ hierarchy" to avoid the ugly "non-remote-tracking branch".
The new formulation actually corresponds to how the code detects this
case (i.e. prefixcmp(refname, "refs/remotes")).

Also, we use 'remote-tracking branch' in generated merge messages (by
merge an fmt-merge-msg).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>