addremove: document default similarity behavior (issue3429)

largefiles: follow normal codepath for addremove if non-largefiles repo (issue3249)

mq: add --no-backup for qpush/qpop/qgoto

mq: backup local changes in qpush --force qpush help says the following about --force: 1- When -f/--force is applied, all local changes in patched files will be lost. 2- Apply on top of local changes In practice, qpush --force will attempt to apply the patch on top of local changes, and on success will merge them in the pushed patch. On failure, patched files will contain a mix of local changes (where the patch could not apply) and a mix of patch changes (were it applied). So, local changes are less lost than entangled with a mass of other changes. This patch makes qpush --force backup all locally modified files touched by the next patch being applied. When multiple patches are being pushed, this logic is repeated for each patch. Note that modified but successfully patched files are preserved as well.