diff: rewrite diffline Make diffline more readable, using strings with placeholders rather than appending to a list from many ifs that makes difficult to understand the actual output format.

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.

diff: move b85diff to mdiff module b85diff generates a binary diff, so we move this code to mdiff module along with unidiff for text diffs. All diffing mechanisms will be in the same place. In an upcoming patch we will remove the responsibility to print the index header from b85diff and move it back to patch, since it's a patch metadata header, not part of the diff generation.