SimilarityRenameDetector: Initialize sizes to 0

SimilarityRenameDetector: Initialize sizes to 0

Setting the array elements to -1 is more expensive than relying on
the allocator to zero the array for us first.  Shifting the code to
always add 1 to the size (so an empty file is actually 1 byte long)
allows us to detect an unloaded size by comparing to 0, thus saving
the array fill calls.

Change-Id: Iad859e910655675b53ba70de8e6fceaef7cfcdd1
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>