disable git worktree support

this caused the git prompt to disappear in the latest version of the
theme bobthefish.

ref: oh-my-fish/theme-bobthefish#196

config/fish/config.fish

@@ -42,7 +42,7 @@ set -g theme_display_git_untracked yes
 set -g theme_display_git_ahead_verbose yes
 set -g theme_display_git_dirty_verbose yes
 set -g theme_display_git_master_branch yes
-set -g theme_git_worktree_support yes
+set -g theme_git_worktree_support no
 set -g theme_display_vagrant no
 set -g theme_display_docker_machine no
 set -g theme_display_k8s_context no