Sync files with the same size but not content

This is done by comparing an MD5 sum of the file with the etag from S3. The etag
from S3 is an md5 of the entire file unless it is a multipart upload.

My assumption is that the filesize comparison was used to prevent downloading or
opening large files. This commit only runs the MD5 comparison for "small files"
(< 50 kilobytes). This is for two reasons: this way we avoid the processing of
claculating an MD5 sum for very large files, and we avoid the issue of dealing
with miltipart uploads. The reasonable assumption is that if a file changes
without it's size changing, it is likely to be a small file. For example, my
use-case is syncing a REVISION file that contains the git revision as a sha1.
This file is always 41 bytes, but changes frequently.

The "small file" is 50 kilobytes for now, but could easily be changed.