git: 'update-cached-checkout' returns the commit relation.

* guix/git.scm (update-cached-checkout): Add #:starting-commit
parameter.  Call 'commit-relation' when #:starting-commit is true.
Always return the relation or #f as the third value.
(latest-repository-commit): Adjust accordingly.
* guix/import/opam.scm (get-opam-repository): Likewise.
* tests/channels.scm ("latest-channel-instances includes channel dependencies")
("latest-channel-instances excludes duplicate channel dependencies"):
Update mock of 'update-cached-checkout' accordingly.

guix/channels.scm

@@ -218,7 +218,7 @@ result is unspecified."
     (and (string=? (basename file) ".git")
          (eq? 'directory (stat:type stat))))
 
-  (let-values (((checkout commit)
+  (let-values (((checkout commit relation)
                 (update-cached-checkout (channel-url channel)
                                         #:ref (channel-reference channel))))
     (when (guix-channel? channel)

guix/git.scm

@@ -262,14 +262,16 @@ definitely available in REPOSITORY, false otherwise."
                                  #:key
                                  (ref '(branch . "master"))
                                  recursive?
+                                 starting-commit
                                  (log-port (%make-void-port "w"))
                                  (cache-directory
                                   (url-cache-directory
                                    url (%repository-cache-directory)
                                    #:recursive? recursive?)))
-  "Update the cached checkout of URL to REF in CACHE-DIRECTORY.  Return two
+  "Update the cached checkout of URL to REF in CACHE-DIRECTORY.  Return three
 values: the cache directory name, and the SHA1 commit (a string) corresponding
-to REF.
+to REF, and the relation of the new commit relative to STARTING-COMMIT (if
+provided) as returned by 'commit-relation'.
 
 REF is pair whose key is [branch | commit | tag | tag-or-commit ] and value
 the associated data: [<branch name> | <sha1> | <tag name> | <string>].
@@ -302,7 +304,17 @@ When RECURSIVE? is true, check out submodules as well, if any."
            (remote-fetch (remote-lookup repository "origin"))))
      (when recursive?
        (update-submodules repository #:log-port log-port))
-     (let ((oid (switch-to-ref repository canonical-ref)))
+
+     ;; Note: call 'commit-relation' from here because it's more efficient
+     ;; than letting users re-open the checkout later on.
+     (let* ((oid      (switch-to-ref repository canonical-ref))
+            (new      (and starting-commit
+                           (commit-lookup repository oid)))
+            (old      (and starting-commit
+                           (commit-lookup repository
+                                          (string->oid starting-commit))))
+            (relation (and starting-commit
+                           (commit-relation old new))))
 
        ;; Reclaim file descriptors and memory mappings associated with
        ;; REPOSITORY as soon as possible.
@@ -310,7 +322,7 @@ When RECURSIVE? is true, check out submodules as well, if any."
                               'repository-close!)
          (repository-close! repository))
 
-       (values cache-directory (oid->string oid))))))
+       (values cache-directory (oid->string oid) relation)))))
 
 (define* (latest-repository-commit store url
                                    #:key
@@ -343,7 +355,7 @@ Log progress and checkout info to LOG-PORT."
 
   (format log-port "updating checkout of '~a'...~%" url)
   (let*-values
-      (((checkout commit)
+      (((checkout commit _)
         (update-cached-checkout url
                                 #:recursive? recursive?
                                 #:ref ref

guix/import/opam.scm

@@ -115,7 +115,7 @@
 (define (get-opam-repository)
   "Update or fetch the latest version of the opam repository and return the
 path to the repository."
-  (receive (location commit)
+  (receive (location commit _)
     (update-cached-checkout "https://github.com/ocaml/opam-repository")
     location))
 

tests/channels.scm

@@ -136,11 +136,11 @@
                    (url "test")))
          (test-dir (channel-instance-checkout instance--simple)))
     (mock ((guix git) update-cached-checkout
-           (lambda* (url #:key ref)
+           (lambda* (url #:key ref starting-commit)
              (match url
-               ("test" (values test-dir "caf3cabba9e"))
+               ("test" (values test-dir "caf3cabba9e" #f))
                (_      (values (channel-instance-checkout instance--no-deps)
-                               "abcde1234")))))
+                               "abcde1234" #f)))))
           (with-store store
             (let ((instances (latest-channel-instances store (list channel))))
               (and (eq? 2 (length instances))
@@ -155,11 +155,11 @@
                    (url "test")))
          (test-dir (channel-instance-checkout instance--with-dupes)))
     (mock ((guix git) update-cached-checkout
-           (lambda* (url #:key ref)
+           (lambda* (url #:key ref starting-commit)
              (match url
-               ("test" (values test-dir "caf3cabba9e"))
+               ("test" (values test-dir "caf3cabba9e" #f))
                (_      (values (channel-instance-checkout instance--no-deps)
-                               "abcde1234")))))
+                               "abcde1234" #f)))))
           (with-store store
             (let ((instances (latest-channel-instances store (list channel))))
               (and (= 2 (length instances))