lastModified field in path docs is incorrect for HG and Git

Since HG and Git are not scanned chronologically, the lastModified time stored in the paths documents in lucene may be incorrect. This is because currently, the lastModified timestamp is set to the modified time of the last changeset containing that file that was indexed.

The quicknav index calculates lastModified the same it seems, and is also incorrect.

One way to fix this for DVCS repos is to do a lookup of the document before it gets added, and re-adding the Max of the new or old last modified timestamp.