Avoid :redir inside execute()

This was happening when starting vim with `let g:gitgutter_log = 1`
and generated E930.

autoload/gitgutter/debug.vim

@@ -75,23 +75,21 @@ function! gitgutter#debug#log(message, ...) abort
       endif
     endif
 
-    execute 'redir >> '.s:log_file
     if s:new_log_session
       let s:start = reltime()
-        silent echo "\n==== start log session ===="
+      call writefile(['==== start log session ===='], s:log_file, 'a')
     endif
 
     let elapsed = reltimestr(reltime(s:start)).' '
-      silent echo ''
+    call writefile([''], s:log_file, 'a')
     " callers excluding this function
-      silent echo elapsed.expand('<sfile>')[:-22].':'
+    call writefile([elapsed.expand('<sfile>')[:-22].':'], s:log_file, 'a')
-      silent echo elapsed.s:format_for_log(a:message)
+    call writefile([elapsed.s:format_for_log(a:message)], s:log_file, 'a')
     if a:0 && !empty(a:1)
       for msg in a:000
-          silent echo elapsed.s:format_for_log(msg)
+        call writefile([elapsed.s:format_for_log(msg)], s:log_file, 'a')
       endfor
     endif
-    redir END
 
     let s:new_log_session = 0
   endif