TagSet: Abort build when the thread is interrupted

When a client disconnects while its ref advertisement is still being
computed, sshd interrupts the thread serving the request. From then
on, every object read from a file-based repository throws
InterruptedIOException.

TagSet.build() catches IOException separately for each ref in
addRef() and addTag(). It logs a warning with a full stack trace and
moves on to the next ref, so an interrupted build of a repository
with many refs writes one warning per ref and burns CPU on reads that
are all bound to fail. On repositories with hundreds of thousands of
branches, a single cancelled fetch or push can flood error_log with
that many warnings.

Rethrow InterruptedIOException from the per-ref handlers so that the
first failed read aborts the whole build. Log it once, at FINE level,
because an interrupt is an expected consequence of the client going
away and not a repository error.

Release-Notes: Fixed a flood of "Error on <ref>" warnings from TagSet when a fetch or push was cancelled while refs were being advertised
Change-Id: Ie0762ddfaf636e4b6aca852f510439e8fb73d337