Arcanist: Fails to parse git log with GPG signatures

If the user has configured log.showSignature in ~/.gitconfig, all invocations of git log will include signature information. This triggers the following bug:

EXCEPTION: (Exception) Diff Parse Exception: Expected 'Author:'.
              1   commit <hash>
   >>>        2   gpg: Signature made Mon 07 Sep 2020 08:32:40 PM EDT
              3   gpg:                using RSA key <key hash>
              4   gpg: Good signature from <name> [ultimate]
 at [<arcanist>/src/parser/ArcanistDiffParser.php:1223]
arcanist(), phutil()
  #0 ArcanistDiffParser::didFailParse(string) called at [<arcanist>/src/parser/ArcanistDiffParser.php:392]
  #1 ArcanistDiffParser::parseCommitMessage(ArcanistDiffChange) called at [<arcanist>/src/parser/ArcanistDiffParser.php:334]
  #2 ArcanistDiffParser::parseDiff(string) called at [<arcanist>/src/repository/api/ArcanistGitAPI.php:1129]
  #3 ArcanistGitAPI::loadWorkingCopyDifferentialRevisions(ConduitClient, array) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:1493]
  #4 ArcanistDiffWorkflow::buildCommitMessage() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:469]
  #5 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:394]

Solution: Pass the “–no-show-signature” flag, introduced in Git 2.10.0, to all invocations of git log in ArcanistGitAPI.php.