magit-file-log: use magit-refresh-log-buffer instead of magit-refresh-file-log-buffer (v2)

I have incorporated your suggestions and "shortened" some doc-strings.

The differences to v1 #948 are:

```diff
diff --git a/magit.el b/magit.el
index 0bd038f..0018ba6 100644
--- a/magit.el
+++ b/magit.el
@@ -5896,9 +5896,7 @@ With a prefix arg, do a submodule update --init."
 
 (magit-define-command file-log (file &optional use-graph)
   "Display the log for the currently visited file or another one.
-
-With a prefix argument or if no file is currently visited, ask
-for the file whose log must be displayed."
+With a prefix argument show the log graph."
   (interactive
    (list (magit-read-file-from-rev (magit-get-current-branch)
                                    (magit-buffer-file-name t))
@@ -5907,7 +5905,8 @@ for the file whose log must be displayed."
                     #'magit-log-mode
                     #'magit-refresh-log-buffer
                     'oneline "HEAD"
-                    (and use-graph (list "--graph"))
+                    `(,@(and use-graph (list "--graph"))
+                      ,@magit-custom-options)
                     file))
 
 (magit-define-command reflog (ref)
@@ -5960,7 +5959,7 @@ from the parent keymap `magit-mode-map' are also available."
                   (list (concat "--pretty=format:%h%d "
                                 (and magit-log-show-gpg-status "%G?")
                                 "[%an][%ar]%s"))))
-             ,@args "--"))))
+             ,@args "--" ,file))))
 
 (defun magit-log-show-more-entries (&optional arg)
   "Grow the number of log entries shown.
```

I also found a problem with the use of `magit-read-file-from-rev`: it should fallback to "HEAD" if REVISION is nil.