Do not deactivate gitgutter when &diff is set.

In #303 there was a request to deactivate gitgutter when &diff is set,
although no reason was given initially.  Then it was reported that
gitgutter showed signs, in the wrong place, during Fugitive's Gdiff.

However I cannot reproduce signs showing in Gdiff's index buffer.  And
the signs in the working tree buffer are correct.

Furthermore the commit which deactivated gitgutter (85f6e136) had the
side-effect of breaking `:redir` and clearing Vim's startup message
(see #500).

Therefore since that change had no benefit, and a couple of drawbacks,
it makes sense to undo it.

Closes #500.

autoload/gitgutter.vim

@@ -45,8 +45,6 @@ function! gitgutter#process_buffer(bufnr, force) abort
       endif
 
     endif
-  else
-    call s:clear(a:bufnr)
   endif
 endfunction
 

autoload/gitgutter/utility.vim

@@ -50,8 +50,7 @@ function! gitgutter#utility#is_active(bufnr) abort
         \ !pumvisible() &&
         \ s:is_file_buffer(a:bufnr) &&
         \ s:exists_file(a:bufnr) &&
-        \ s:not_git_dir(a:bufnr) &&
-        \ !s:vimdiff(a:bufnr)
+        \ s:not_git_dir(a:bufnr)
 endfunction
 
 function! s:not_git_dir(bufnr) abort
@@ -206,28 +205,6 @@ function! s:strip_trailing_new_line(line) abort
   return substitute(a:line, '\n$', '', '')
 endfunction
 
-" Returns 1 if any of the given buffer's windows has the `&diff` option set,
-" or 0 otherwise.
-if exists('*win_findbuf')
-  function! s:vimdiff(bufnr) abort
-    for winid in win_findbuf(a:bufnr)
-      if getwinvar(winid, '&diff')
-        return 1
-      endif
-    endfor
-    return 0
-  endfunction
-else
-  function! s:vimdiff(bufnr) abort
-    for winnr in range(1, winnr('$'))
-      if winbufnr(winnr) == a:bufnr && getwinvar(winnr, '&diff')
-        return 1
-      endif
-    endfor
-    return 0
-  endfunction
-endif
-
 function! s:windows()
   return has('win64') || has('win32') || has('win16')
 endfunction