* Handle "-" when found as length field in fetch.txt, per bagit spec.
BDBag can read and resolve files in bags which have unspecified content
lengths, but will not allow them to be created via remote-file-manifests
(because Payload-0xum cannot be reliably determined without byte counts
for all referenced files), and will throw an exception during the
creating/updating of a bag where an unspecified length is encountered.
* Fix duplicate manifest entry issue when creating/updating bags that
have remote file references for payload files that are already present
in the bag. It is now a conflict for a bag to have both a file in the
local payload and in fetch.txt during create/update, and an exception
will be thrown when this condition is detected.
* Ensure URL escaping in fetch.txt, per bagit spec.
* Don't emit blank lines when CLI is in quiet mode.