Check whether inside git repository then display warning

src/compiler/crystal/tools/doc/generator.cr

@@ -20,6 +20,9 @@ class Crystal::Doc::Generator
     @types = {} of Crystal::Type => Doc::Type
     @repo_name = ""
     compute_repository
+    if @repo_name == ""
+      puts "WARNING there is not inside git repository. some features are not available."
+    end
     @is_crystal_repo = @repo_name == "github.com/crystal-lang/crystal"
   end
 
@@ -278,6 +281,10 @@ class Crystal::Doc::Generator
   end
 
   def compute_repository
+    # check whether inside git work-tree
+    `git rev-parse --is-inside-work-tree >/dev/null 2>&1`
+    return unless $?.success?
+
     remotes = `git remote -v`
     return unless $?.success?