Change: (magit-todos-branch-list-merge-base-ref) Detect automatically

Fixes #151.  Closes #152.

Reported-by: Aaron Zeng <https://github.com/bcc32>

README.org

@@ -133,6 +133,7 @@ Helm and Ivy are also supported.  Note that the =helm= and =ivy= packages are no
 
 *Changed*
 + Improve behavior when scanner backend exits with an error.  (Now an error is signaled and the command's output is shown.)
++ Option ~magit-todos-branch-list-merge-base-ref~ defaults to nil, which automatically detects the default branch name using function ~magit-main-branch~. ([[https://github.com/alphapapa/magit-todos/issues/151][#151]].  Thanks to [[https://github.com/bcc32][Aaron Zeng]] for reporting.)
 
 *Fixed*
 + Updated ~find|grep~ scanner for newer versions of GNU ~find~ that interpret some arguments differently.  (Tested on v4.8.0.)

magit-todos.el

@@ -370,7 +370,7 @@ This can be toggled locally in Magit buffers with command
                  (const :tag "In non-master branches" branch)
                  (const :tag "Always" t)))
 
-(defcustom magit-todos-branch-list-merge-base-ref "master"
+(defcustom magit-todos-branch-list-merge-base-ref nil
   "Commit ref passed to command \"git merge-base HEAD\".
 Determines the ancestor commit from which the current branch's
 todos should be searched for.  May be overridden in the case that
@@ -387,7 +387,8 @@ By default, the branch todo list would show todos from both the
 \"topic\" branch and the \"topic2\" branch.  To show only todos
 from the \"topic2\" branch, this option could be set to
 \"topic\"."
-  :type 'string)
+  :type '(choice (const :tag "Automatic" :doc "Value returned by `magit-main-branch'" nil)
+                 (string :tag "Specified branch name")))
 
 (defcustom magit-todos-submodule-list nil
   "Show submodule to-do list."
@@ -760,12 +761,14 @@ This function should be called from inside a ‘magit-status’ buffer."
      (magit-todos--insert-items (current-buffer) magit-todos-item-cache)))
   (when (or (eq magit-todos-branch-list t)
             (and (eq magit-todos-branch-list 'branch)
-                 (not (string= "master" (magit-get-current-branch)))))
+                 (not (equal (or magit-todos-branch-list-merge-base-ref (magit-main-branch))
+                             (magit-get-current-branch)))))
     ;; Insert branch-local items.
     (magit-todos--scan-with-git-diff :magit-status-buffer (current-buffer)
                                      :directory default-directory
                                      :depth magit-todos-depth
-                                     :heading (format "TODOs (branched from %s)" magit-todos-branch-list-merge-base-ref))))
+                                     :heading (format "TODOs (branched from %s)"
+                                                      (or magit-todos-branch-list-merge-base-ref (magit-main-branch))))))
 
 (cl-defun magit-todos--insert-items (magit-status-buffer items &key branch-p)
   "Insert to-do ITEMS into MAGIT-STATUS-BUFFER.