diff: swap and simplify diffline args Args order swapped, since a, b are more important than revs (which is only used on non-git format), and change to read opts from context.

diff: change how quiet mode supresses diffline Before, quiet mode produced no diffline header for mercurial as a side effect of not populating "revs". This was a weird side effect, and we will always need revs for git index header that will be added in upcoming patches, so now we just check ui.quiet from diffline directly.

diff: move diffline to patch module diffline is not part of diff computation, so it makes more sense to place it with other header generation in patch module. In upcoming patches we will generalize this approach for all headers added in the patch, including the git index header.

diff: unify calls to diffline diffline was called from trydiff for binary diffs and from unidiff for text diffs. In this patch we unify those calls into one. diffline is also a header, not part of diff mechanisms, so it makes sense to remove that responsibility from the mdiff module. In upcoming patches we will move diffline to patch module and keep grouping responsibilities.