1 files changed, 20 insertions, 0 deletions
diff --git a/lisp/init-git.el b/lisp/init-git.el
index 29525a5..d6ac3c9 100644
--- a/lisp/init-git.el
+++ b/lisp/init-git.el
@@ -444,6 +444,26 @@ function."
   (when (derived-mode-p 'magit-log-mode)
     (magit-diff-show-or-scroll-up)))
 
+(defun km/magit-rev-ancestor-p (rev-a rev-b)
+  "Report whether REV-A is the ancestor of REV-B.
+Use the revision at point as REV-B.  With prefix argument or if
+there is no revision at point, prompt for the revision.  Always
+prompt for REV-A."
+  (interactive
+   (let* ((atpoint (or (and magit-blame-mode (magit-blame-chunk-get :hash))
+                       (magit-branch-or-commit-at-point)
+                       (magit-tag-at-point)))
+          (commit (or (and (not current-prefix-arg) atpoint)
+                      (magit-read-branch-or-commit "Descendant" atpoint))))
+     (list (magit-read-other-branch-or-commit
+            (format "Test if ancestor of %s" commit) commit)
+           commit)))
+  (message "%s is %san ancestor of %s" rev-a
+           (if (magit-git-success "merge-base" "--is-ancestor"
+                                  rev-a rev-b)
+               "" "NOT ")
+           rev-b))
+
 (define-key ctl-x-4-map "g" 'magit-find-file-other-window)
 (define-key km/file-map "g" 'magit-find-file)