git: Add ‘tag->commit’ and use it in (guix channels).

* guix/git.scm (tag->commit): New procedure, taken from…
(resolve-reference): … here.  Use it in the ‘tag’ case.
* guix/channels.scm (resolve-channel-news-entry-tag): Use ‘tag->commit’
instead of custom code.

Change-Id: I46ea387345dc1b695ce0702991a52d0cde29e2f0

guix/channels.scm

@@ -26,6 +26,7 @@
                            commit-difference
                            repository-info
                            commit-short-id
+                           tag->commit
                            with-repository)
   #:autoload   (guix git-authenticate) (authenticate-repository)
   #:autoload   (guix openpgp) (openpgp-public-key-fingerprint
@@ -1148,14 +1149,8 @@ the field its 'tag' refers to.  A 'git-error' exception is raised if the tag
 cannot be found."
   (if (channel-news-entry-commit entry)
       entry
-      (let* ((tag       (channel-news-entry-tag entry))
-             (reference (reference-lookup repository
-                                          (string-append "refs/tags/" tag)))
-             (target    (reference-target reference))
-             (oid       (let ((obj (object-lookup repository target)))
-                          (if (= OBJ-TAG (object-type obj)) ;annotated tag?
-                              (tag-target-id (tag-lookup repository target))
-                              target))))
+      (let* ((tag (channel-news-entry-tag entry))
+             (oid (object-id (tag->commit repository tag))))
         (channel-news-entry (oid->string oid) tag
                             (channel-news-entry-title entry)
                             (channel-news-entry-body entry)))))

guix/git.scm

@@ -68,6 +68,7 @@
             commit-descendant?
             commit-id?
             commit-short-id
+            tag->commit
 
             remote-refs
 
@@ -237,6 +238,19 @@ is a tag name.  This is based on a simple heuristic so use with care!"
 (define commit-short-id
   (compose (cut string-take <> 7) oid->string commit-id))
 
+(define (tag->commit repository tag)
+  "Resolve TAG in REPOSITORY and return the corresponding object, usually a
+commit."
+  (let* ((oid (reference-name->oid repository
+                                   (string-append "refs/tags/" tag)))
+         (obj (object-lookup repository oid)))
+    ;; OID may designate an "annotated tag" object or a "commit" object.
+    ;; Return the commit object in both cases.
+    (if (= OBJ-TAG (object-type obj))
+        (object-lookup repository
+                       (tag-target-id (tag-lookup repository oid)))
+        obj)))
+
 (define (resolve-reference repository ref)
   "Resolve the branch, commit or tag specified by REF, and return the
 corresponding Git object."
@@ -283,15 +297,7 @@ corresponding Git object."
                   ;; There's no such tag, so it must be a commit ID.
                   (resolve `(commit . ,str)))))))
       (('tag    . tag)
-       (let* ((oid (reference-name->oid repository
-                                        (string-append "refs/tags/" tag)))
-              (obj (object-lookup repository oid)))
-         ;; OID may designate an "annotated tag" object or a "commit" object.
-         ;; Return the commit object in both cases.
-         (if (= OBJ-TAG (object-type obj))
-             (object-lookup repository
-                            (tag-target-id (tag-lookup repository oid)))
-             obj))))))
+       (tag->commit repository tag)))))
 
 (define (switch-to-ref repository ref)
   "Switch to REPOSITORY's branch, commit or tag specified by REF.  Return the