skip indentation for single line comments

In certain cases (like visual block mode) the indentation rules can
apply before the syntax updates, resulting in incorrect indentation and
weird insertion behaviour.

Fixes NixOS#13

indent/nix.vim

@@ -31,6 +31,12 @@ function! GetNixIndent()
     return 0
   endif
 
+  " Skip indentation for single line comments explicitly, in case a
+  " comment was just inserted (eg. visual block mode)
+  if getline(v:lnum) =~ '^\s*#'
+    return indent(v:lnum)
+  endif
+
   if synIDattr(synID(v:lnum, 1, 1), "name") !~ s:skip_syntax
     let current_line = getline(v:lnum)
     let last_line = getline(lnum)