match: skip walking up the directory hierarchy if the number of pats are small Previously, we would receive a path like abc/def/ghi and "walk up" the directory hierarchy, checking abc/def, abc, and `b''` to see if they were in the set of prefixes that this matcher covered. We did this indiscriminately - we generated all of these paths even if the set of prefixes the matcher covered was completely empty, which is the case for a lot of repos at my company (the narrow matcher we use is usually non-recursive). This brings the time for a rebase in one of my repos from 12.20s to 10.87s. In this particular repo, this is entirely due to the `len(prefix_set) == 0` check, as I do not have any recursive patterns in the narrowspec. Differential Revision: https://phab.mercurial-scm.org/D9488