Add simple logging.

autoload/gitgutter/debug.vim

@@ -1,3 +1,5 @@
+let s:log_file = expand('<sfile>:p:h:h:h').'/'.'gitgutter.log'
+
 function! gitgutter#debug#debug()
   " Open a scratch buffer
   vsplit __GitGutter_Debug__
@@ -70,8 +72,19 @@ function! gitgutter#debug#output(text)
   call append(line('$'), a:text)
 endfunction
 
-function! gitgutter#debug#log(message)
-  let msg = type(a:message) == 1 ? split(a:message, '\n') : a:message
-  call writefile(msg, 'gitgutter.log', 'a')
+" assumes optional args are calling function's optional args
+function! gitgutter#debug#log(message, ...)
+  if g:gitgutter_log
+    execute 'redir >> '.s:log_file
+      " callers excluding this function
+      silent echo "\n".expand('<sfile>')[:-22].':'
+      silent echo type(a:message) == 1 ? join(split(a:message, '\n'),"\n") : a:message
+      if a:0 && !empty(a:1)
+        for msg in a:000
+          silent echo type(msg) == 1 ? join(split(msg, '\n'),"\n") : msg
+        endfor
+      endif
+    redir END
+  endif
 endfunction
 

autoload/gitgutter/diff.vim

@@ -134,9 +134,11 @@ function! gitgutter#diff#run_diff(realtime, preserve_full_diff)
           \ 'on_stderr': function('gitgutter#handle_diff_job'),
           \ 'on_exit':   function('gitgutter#handle_diff_job')
           \ })
+    call gitgutter#debug#log('[job_id: '.job_id.'] '.cmd)
     if job_id < 1
       throw 'diff failed'
     endif
+
     call gitgutter#utility#pending_job(job_id)
     return 'async'
   else

autoload/gitgutter/utility.vim

@@ -121,6 +121,8 @@ function! gitgutter#utility#using_xolox_shell()
 endfunction
 
 function! gitgutter#utility#system(cmd, ...)
+  call gitgutter#debug#log(a:cmd, a:000)
+
   if gitgutter#utility#using_xolox_shell()
     let options = {'command': a:cmd, 'check': 0}
     if a:0 > 0