replaced references to `git` with `command git`, to make the shell faster for git wrapper users

fish_prompt.fish

 function _git_branch_name
-  echo (git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
+  echo (command git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
 end
 
 function _is_git_dirty
-  echo (git status -s --ignore-submodules=dirty ^/dev/null)
+  echo (command git status -s --ignore-submodules=dirty ^/dev/null)
 end
 
 function fish_prompt