buildhistory: Restore parsing time optimalization

buildhistory: Restore parsing time optimalization

* it was added in:
  http://git.openembedded.org/openembedded-core/commit/meta/classes/buildhistory.bbclass?id=540a2a30be21c3eca4323efbe91e7dcfc31a4c97
* then accidentally removed in:
  http://git.openembedded.org/openembedded-core/commit/meta/classes/buildhistory.bbclass?id=6db6c74f2876df7a183990426bb4262972966441

(From OE-Core rev: cbd868c34718e8fd2a41f5382da71ee959ea4e71)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>