Update arcanist to rev 4 via SR 798129
    
    https://build.opensuse.org/request/show/798129
    by user guoyunhe + dimstar_suse
    - Update to version 0.0~git.20200426T084008~b81818b2:
      * Accommodate PHP 7.4 changes to certain "preg_match_all()" calls
      * Make "arc <workflow> --help" work again for workflows which haven't updated yet
      * Allow HTTPFuture callers to disable processing of "Content-Encoding" response headers
      * If the Conduit server asserts it has the "gzip" capability, compress requests
      * If the Conduit client supports gzip, make calls with "Accept-Encoding: gzip"
      * In "phutil_loggable_string()", encode every byte above 0x7F
      * Only inject legacy Arcanist workflows into "help" if run from the context of an Arcanist runtime
      * Update "arc paste" for Toolsets
      * Crudely bridge legacy workflows into "arc help"
      * Remove "backout", "close", "flag", "start", "stop", "time", and "revert" workflows