fish: functions: Execute fast-tags only if in a git project

In neovim, we run this after saving a file. However, when opening a
Haskell file which is outside of a project or in a directory which
has all other Haskell code, it would generate tags for every project
in this directory blocking neovim in the mean time.

fish/.config/fish/functions/fasttags.fish

@@ -1,7 +1,9 @@
 function fasttags --description "Generate tags for Haskell project"
-    if not test -e tags
-        fast-tags --fully-qualified -R .
-    else
-        fast-tags --fully-qualified -R $argv
+    if test -d .git
+        if not test -e tags
+            fast-tags --fully-qualified -R .
+        else
+            fast-tags --fully-qualified -R $argv
+        end
     end
 end