Sped up view branch construction by 50%

Sped up view branch construction by 50%

A benchmark in my sound repository with `git-annex view feedtitle=*`
took 2:52 wall clock time before and 1:58 after. Though it still only used
130% of CPU.

This is the same kind of optimisation that is in seekFilteredKeys, though
that precaches location logs while this streams the metadata logs direct
to parsing them.

seekFilteredKeys contains more streaming, to find the annexed files, and
this could be further sped up with similar streaming.

Sponsored-by: Nicholas Golder-Manning on Patreon