Closes #3: Remove dependency of git plugin from robbyrussell theme

plugins/git/git_branch_name.fish

-function git_branch_name
-  echo (git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
-end

plugins/git/is_git_dirty.fish

-function is_git_dirty
-  echo (git status -s --ignore-submodules=dirty ^/dev/null)
-end

themes/robbyrussell/fish_prompt.fish

+function _git_branch_name
+  echo (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)
+end
+
 function fish_prompt
   set -l cyan (set_color -o cyan)
   set -l yellow (set_color -o yellow)
@@ -8,11 +16,11 @@ function fish_prompt
   set -l arrow "$red➜ "
   set -l cwd $cyan(basename (prompt_pwd))
 
-  if [ (git_branch_name) ]
-    set -l git_branch $red(git_branch_name)
+  if [ (_git_branch_name) ]
+    set -l git_branch $red(_git_branch_name)
     set git_info "$blue git:($git_branch$blue)"
 
-    if [ (is_git_dirty) ]
+    if [ (_is_git_dirty) ]
       set -l dirty "$yellow ✗"
       set git_info "$git_info$dirty"
     end