ref-filter: add braces to if/else if/else chain

ref-filter: add braces to if/else if/else chain

Per the CodingGuidelines add braces to an if/else if/else chain where
only the "else" had braces. This is in preparation for a subsequent
change where the "else if" will have lines added to it.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>