Optimize getline(istream&, fbstring&) implementation

Optimize getline(istream&, fbstring&) implementation

Summary:
Current `getline` implementation in `fbstring` always allocates, even if the passed string is already large enough. Furthermore, the growing strategy relies on outdated assumptions about the allocator.
This implementation reuses the existing allocation as much as possible, then uses exponential growth.

Reviewed By: luciang, philippv

Differential Revision: D2871976

fb-gh-sync-id: 8db9512030be3f4953efa8f008747827504c032c