cat: increase perf when catting single files Special case the single file case in hg cat. This allows us to avoid parsing the manifest, which shaves 15% off hg cat perf. This is worth it, since automation often uses hg cat for retrieving single files.

changectx: increase perf of walk function When running 'hg cat -r . <file>' it was doing an expensive ctx.walk(m) which applied the regex to every file in the manifest. This changes changectx.walk to iterate over just the files in the regex, if no other patterns are specified. This cuts hg cat time by 50% in our repo and probably benefits a few other commands as well.