[PATCH] Support adding Message-Id to attached patches

[Kyle Meyer <kyle@kyleam.com>]

With an inline patch that has a Message-Id, this information can be
linked up to the patch when applied (e.g., with git-am's --message-id
flag or using a post-applypatch hook [1]).  Unfortunately, this method
fails for projects where it is common to attach patches, as there is
no Message-Id within the patch.

Teach piem-am-ready-mbox how to insert the Message-Id that piem-mid
reports, which should always correspond to the message that contains
the patch attachments.

[1] Here's an example that used to keep the commit->Message-Id mapping
    in git.git:
    https://lore.kernel.org/git/xmqq7e5ag4g5.fsf@gitster-ct.c.googlers.com/
---
 piem.el | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/piem.el b/piem.el
index a757092..fcd5d8c 100644
--- a/piem.el
+++ b/piem.el
@@ -101,6 +101,14 @@ (defcustom piem-am-ready-mbox-functions nil
 arguments and inserts the mbox's contents in the current buffer."
   :type 'hook)
 
+(defcustom piem-add-message-id-header nil
+  "Whether to add Message-Id header to non-mail patches.
+If this value is non-nil and a patch returned by a function in
+`piem-am-ready-mbox-functions' looks like a patch that was
+attached rather than sent inline, add a Message-Id header with
+the value of `piem-mid'."
+  :type 'boolean)
+
 (defcustom piem-git-executable
   (or (and (boundp 'magit-git-executable) magit-git-executable)
       "git")
@@ -300,10 +308,19 @@ (defun piem-am-ready-mbox ()
   (when-let ((fn (run-hook-with-args-until-success
                   'piem-am-ready-mbox-functions)))
     (let ((buffer (generate-new-buffer " *piem am-ready mbox*"))
+          (mid (and piem-add-message-id-header (piem-mid)))
           has-content)
       (with-current-buffer buffer
         (funcall fn)
-        (setq has-content (< 1 (point-max))))
+        (setq has-content (< 1 (point-max)))
+        (when (and has-content mid)
+          (goto-char (point-min))
+          (while (re-search-forward
+                  (rx "From " (>= 40 hex-digit) " Mon Sep 17 00:00:00 2001")
+                  nil t)
+            (search-forward "\n\n")
+            (forward-line -1)
+            (insert (format "Message-Id: <%s>\n" mid)))))
       (if has-content
           buffer
         (kill-buffer buffer)

base-commit: 54ea4ed7d1bc64222cf837ecc046f7aa4f42c769
-- 
2.26.2