Improvement fbvector.computePushBackCapacity

Improvement fbvector.computePushBackCapacity

Summary:
See the github pull request at https://github.com/facebook/folly/issues/135

Important optimizations: remove one of the branches and change ##empty()## to ##capacity()==0##

Test Plan:
Ran extended fbvector test suite (re-enabled in experimental/njormrod)

Also

fbconfig -r folly && fbmake runtests

Reviewed By: markisaa@fb.com

Subscribers: trunkagent, sdwilsh, njormrod, folly-diffs@, yfeldblum

FB internal diff: D1869112

Tasks: 6338531

Signature: t1:1869112:1424823901:d2d7331aef82edad1e8c159005cc1c7185550d0c