[RFC PATCH 12/14] am, b4-am: Add commands that extend an existing branch

[Kyle Meyer <kyle@kyleam.com>]

piem-am-create and piem-b4-am-from-mid-create require the caller to
create a new branch or to work on top of a detached head.  Sometimes
it's useful to instead apply a patch to an existing branch (e.g.,
because the caller wants a patch to go directly to the mainline or
because a new patch came in that goes on top of a current topic
branch).  In these cases, an alternative approach is to apply to a
detached head or a temporary branch and then merge into the target.

Although that approach is fairly straightforward (particularly with
the help of magit-merge-into), it might not be obvious to callers.
Add dedicated commands to expose this mode of operation.
---
 piem-b4.el | 14 ++++++++++++++
 piem.el    | 28 ++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/piem-b4.el b/piem-b4.el
index 48836eb2..f949f963 100644
--- a/piem-b4.el
+++ b/piem-b4.el
@@ -175,6 +175,20 @@ (defun piem-b4-am-from-mid-create (mid &optional args toggle-worktree)
       (when clean-fn
         (funcall clean-fn)))))
 
+(defun piem-b4-am-from-mid-extend (mid &optional args toggle-worktree)
+  "Like `piem-b4-am-from-mid-create', but extend an existing branch."
+  (interactive (list (or (piem-mid)
+                         (read-string "Message ID: "))
+                     (transient-args 'piem-b4-am)))
+  (piem-b4-am--check-args args)
+  (pcase-let* ((coderepo (piem-inbox-coderepo-maybe-read))
+               (`(,_ ,mbox-file ,clean-fn)
+                (piem-b4--get-am-files mid coderepo args)))
+    (unwind-protect
+        (piem-am-extend mbox-file nil coderepo toggle-worktree)
+      (when clean-fn
+        (funcall clean-fn)))))
+
 (transient-define-argument piem-b4-am:--outdir ()
   :description "Output directory"
   :class 'transient-option
diff --git a/piem.el b/piem.el
index c3c2dfdf..73e331cc 100644
--- a/piem.el
+++ b/piem.el
@@ -1003,6 +1003,34 @@ (defun piem-am-create (mbox &optional format coderepo toggle-worktree info)
                           (list base)))))
     (piem--git-am mbox format am-directory)))
 
+(defun piem-am-extend (mbox &optional format coderepo toggle-worktree)
+  "Like `piem-am-create', but extend an existing branch."
+  (interactive (piem-am--arguments))
+  (let* ((default-directory (or coderepo default-directory))
+         (am-directory default-directory)
+         (use-worktree (xor piem-am-create-worktree toggle-worktree))
+         (branch (if (and piem-use-magit
+                          (fboundp 'magit-get-current-branch)
+                          (fboundp 'magit-read-branch-or-commit))
+                     (let ((current-branch (magit-get-current-branch))
+                           (b (magit-read-branch-or-commit "Branch")))
+                       (and (not (equal b current-branch)) b))
+                   (read-string "Branch (empty for current): "))))
+    (cond
+     ((and branch (not (string-empty-p branch)))
+      (when use-worktree
+        (setq am-directory
+              (piem-am--read-worktree default-directory branch)))
+      (apply #'piem-process-call nil piem-git-executable
+             (append (if use-worktree
+                         (list "worktree" "add")
+                       (list "checkout"))
+                     (and use-worktree (list am-directory))
+                     (list branch))))
+     (use-worktree
+      (user-error "Worktree can't be created with coderepo's current branch")))
+    (piem--git-am mbox format am-directory)))
+
 \f
 ;;;;; Patch editing
 
-- 
2.34.0