reflog: move "usage" variables and use macros

reflog: move "usage" variables and use macros

Move the "usage" variables in builtin/reflog.c to the top of the file,
in preparation for later commits defining a common "reflog_usage" in
terms of some of these strings, as was done in
8757b35d443 (commit-graph: define common usage with a macro,
2021-08-23).

While we're at it let's make them "const char *const", as is the
convention with these "usage" variables.

The use of macros here is a bit odd, but in subsequent commits we'll
make these use the same pattern as builtin/commit-graph.c uses since
8757b35d443 (commit-graph: define common usage with a macro,
2021-08-23).

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