[split] Move robbyrussell theme to oh-my-fish/theme-robbyrussell

https://github.com/oh-my-fish/theme-robbyrussell

themes/robbyrussell/README.md

-## robbyrussell
-The default theme – cloned from oh-my-zsh.
-
-![robbyrussell theme](https://bachue.github.io/oh-my-fish/images/robbyrussell-screenshot.png)
-
-
-#### Characteristics
-
-* Displays git information in the command prompt when available.
-* If the last command was failed, the indicator would be red, otherwise it's green

themes/robbyrussell/fish_prompt.fish

-# name: RobbyRussel
-#
-# You can override some default options in your config.fish:
-#   set -g theme_display_git_untracked no
-
-function _git_branch_name
-  echo (command git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
-end
-
-function _is_git_dirty
-  set -l show_untracked (git config --bool bash.showUntrackedFiles)
-  set untracked ''
-  if [ "$theme_display_git_untracked" = 'no' -o "$show_untracked" = 'false' ]
-    set untracked '--untracked-files=no'
-  end
-  echo (command git status -s --ignore-submodules=dirty $untracked ^/dev/null)
-end
-
-function fish_prompt
-  set -l last_status $status
-  set -l cyan (set_color -o cyan)
-  set -l yellow (set_color -o yellow)
-  set -l red (set_color -o red)
-  set -l blue (set_color -o blue)
-  set -l green (set_color -o green)
-  set -l normal (set_color normal)
-
-  if test $last_status = 0
-      set arrow "$green➜ "
-  else
-      set arrow "$red➜ "
-  end
-  set -l cwd $cyan(basename (prompt_pwd))
-
-  if [ (_git_branch_name) ]
-    set -l git_branch $red(_git_branch_name)
-    set git_info "$blue git:($git_branch$blue)"
-
-    if [ (_is_git_dirty) ]
-      set -l dirty "$yellow ✗"
-      set git_info "$git_info$dirty"
-    end
-  end
-
-  echo -n -s $arrow ' ' $cwd $git_info $normal ' '
-end
-