This adds different commands to the tiletool.

tiletool dump N

will dump the last N commits.

tiletool md5 GITHASH N

will return the MD5 hash of N commits with GITHASH identifying the last one.

This allows to easily compare local tile stores with tile stores on the server.

Currently this only works the for "golden" tiles.
The next CL will make it generic and more user friendly.

BUG=skia:2930
TBR=jcgregorio

Review URL: https://codereview.chromium.org/773613002