git: 'commit-difference' takes a list of excluded commits.

* guix/git.scm (commit-closure): Add 'visited' optional parameter.
(commit-difference): Add 'excluded' optional parameter; pass second
argument to 'commit-closure'.
* tests/git.scm ("commit-difference, excluded commits"): New test.

guix/git.scm

@@ -347,10 +347,11 @@ Log progress and checkout info to LOG-PORT."
 ;;; Commit difference.
 ;;;
 
-(define (commit-closure commit)
-  "Return the closure of COMMIT as a set."
+(define* (commit-closure commit #:optional (visited (setq)))
+  "Return the closure of COMMIT as a set.  Skip commits contained in VISITED,
+a set, and adjoin VISITED to the result."
   (let loop ((commits (list commit))
-             (visited (setq)))
+             (visited visited))
     (match commits
       (()
        visited)
@@ -360,15 +361,16 @@ Log progress and checkout info to LOG-PORT."
            (loop (append (commit-parents head) tail)
                  (set-insert head visited)))))))
 
-(define (commit-difference new old)
+(define* (commit-difference new old #:optional (excluded '()))
   "Return the list of commits between NEW and OLD, where OLD is assumed to be
-an ancestor of NEW.
+an ancestor of NEW.  Exclude all the commits listed in EXCLUDED along with
+their ancestors.
 
 Essentially, this computes the set difference between the closure of NEW and
 that of OLD."
   (let loop ((commits (list new))
              (result '())
-             (visited (commit-closure old)))
+             (visited (commit-closure old (list->setq excluded))))
     (match commits
       (()
        (reverse result))

tests/git.scm

@@ -96,4 +96,30 @@
              (lset= eq? (commit-difference master4 master2)
                     (list master4 merge master3 devel1 devel2)))))))
 
+(unless (which (git-command)) (test-skip 1))
+(test-assert "commit-difference, excluded commits"
+  (with-temporary-git-repository directory
+      '((add "a.txt" "A")
+        (commit "first commit")
+        (add "b.txt" "B")
+        (commit "second commit")
+        (add "c.txt" "C")
+        (commit "third commit")
+        (add "d.txt" "D")
+        (commit "fourth commit")
+        (add "e.txt" "E")
+        (commit "fifth commit"))
+    (with-repository directory repository
+      (let ((commit1 (find-commit repository "first"))
+            (commit2 (find-commit repository "second"))
+            (commit3 (find-commit repository "third"))
+            (commit4 (find-commit repository "fourth"))
+            (commit5 (find-commit repository "fifth")))
+        (and (lset= eq? (commit-difference commit4 commit1 (list commit2))
+                    (list commit3 commit4))
+             (lset= eq? (commit-difference commit4 commit1 (list commit3))
+                    (list commit4))
+             (lset= eq? (commit-difference commit4 commit1 (list commit5))
+                    (list commit2 commit3 commit4)))))))
+
 (test-end "git")