meta: Add git branch information to fish prompt

If I stayed on zsh instead of switch to fish, I would've probably never
known how easy it is to add certain information to the prompt. Since
the prompt itself is just a function, you can run any commands you want
inside it to get information, including git commands.

This commit adds the current branch you're on only when inside a git
repository, and only when you're not in a tty.

fish/.config/fish/functions/fish_prompt.fish

@@ -12,12 +12,19 @@ function fish_prompt
         set_color magenta;  echo -n "($PWD)"
     else
         set pwd (basename $PWD)
+        set branch (git branch ^/dev/null | sed -n '/\* /s///p')
 
         if [ $pwd = $USER ]
             set pwd "~"
         end
 
         set_color magenta;  echo -n "$pwd"
+
+        if [ "$branch" ]
+            set_color normal;   echo -n " on "
+            set_color yellow;   echo -n "$branch"
+        end
+
         set_color normal;   echo -n " "
         set_color red;      echo -n "➤"
         set_color green;    echo -n "➤"