ref-filter: resolve HEAD when parsing %(HEAD) atom

If the user asks to display (or sort by) the %(HEAD) atom,
ref-filter has to compare each refname to the value of HEAD.
We do so by resolving HEAD fresh when calling populate_value()
on each ref. If there are a large number of refs, this can
have a measurable impact on runtime.

Instead, let's resolve HEAD once when we realize we need the
%(HEAD) atom, allowing us to do a simple string comparison
for each ref. On a repository with 3000 branches (high, but
an actual example found in the wild) this drops the
best-of-five time to run "git branch >/dev/null" from 59ms
to 48ms (~20% savings).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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