mq: write headers for new HG patches in the same order as export (BC)

mq: correctly make an empty line after description in new patches There would in some cases be an empty line between headers and the description - that does not seem right. There should also be an empty line between description and diff - but that was missing. These two mistakes would sometimes make it up for each other so we fix both at once to just show the improvement. Instead of writing an extra newline when writing a header line, write an extra line when it not is written as a part of the description but is necessary anyway.

largefiles: extract 'orig' method in reposetup.status

largefiles: extract 'orig' method in lfilesctx.filectx

largefiles: extract 'orig' method in lfilesmanifestdict.__contains__

largefiles: reduce indentation by dropping 'else' block after 'return'

largefiles: remove dead assignment left over from change 1ff42ee98446