Enlarge log size cap when printing file summary

Summary:
Now the file summary is too small for printing. Enlarge it.
To enable it, allow to pass a size to log buffer.

Test Plan:
Add a unit test.
make all check

Reviewers: ljin, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21723