bump prompt/external/pure

* prompt/external/pure 0421252...dec0253 (26):
  > Update oh-my-zsh instructions
  > 1.2.0
  > Bump zsh-async to 1.0.0, prevents mixed stdout/stderr
  > Merge pull request sorin-ionescu#153 from sindresorhus/preprompt-update-fix
  > Merge pull request sorin-ionescu#149 from mafredri/pure-nitro
  > Close sorin-ionescu#147 PR: Preserve preprompt on Ctrl+L. Fixes sorin-ionescu#145
  > Remove cr from prompt_opts, fixes sorin-ionescu#127
  > Merge pull request sorin-ionescu#142 from zmwangx/string-length-fix
  > Merge pull request sorin-ionescu#144 from zmwangx/rename-prompt-to-preprompt
  > Default to 0 for git rev-list left and right. Fixes sorin-ionescu#137.
  > Close sorin-ionescu#140 PR: Add git arrows customization. Fixes sorin-ionescu#139.
  > 1.1.1
  > Disable prompt expansion for running command
  > Merge pull request sorin-ionescu#130 from zmwangx/rename-variables-for-readability
  > Use standard `[[ ]]` for conditional and add clarifications
  > fix: do the PURE_GIT_PULL check in the correct place
  > 1.1.0
  > Close sorin-ionescu#124 PR: Show hostname in terminal title if session is over ssh.
  > Merge pull request sorin-ionescu#125 from sindresorhus/dirty-check
  > import bug-fix release from zsh-async, fixes async job flushing
  > readme: faq clarificaitons
  > readme: add zpty error to faq with explanation and potential solutions
  > readme: update instructions for antigen and oh-my-zsh. remove incompatible async.plugin.zsh
  > use cd -q to prevent hooks from firing
  > prevent git status leakage when testing if dirty
  > fix paths that are split due to spaces in directory names