remote.c: add braces in anticipation of a follow-up change

remote.c: add braces in anticipation of a follow-up change

The CodingGuidelines say "When there are multiple arms to a
conditional and some of them require braces, enclose even a single
line block in braces for consistency.". Fix the code in
match_explicit() to conform.

While I'm at it change the if/else if/else in guess_ref() to use
braces. This is not currently needed, but a follow-up change will add
a new multi-line condition to that logic.

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