git: 'update-cached-checkout' recognizes truncated commit IDs.

Fixes a regression introduced in
602527ab97 that broke
'tests/guix-build-branch.sh'.

* guix/git.scm (resolve-reference): Revert change from
602527ab97.

guix/git.scm

@@ -261,7 +261,8 @@ corresponding Git object."
                           #f))
                      (_ #f)))
               => (lambda (commit) (resolve `(commit . ,commit))))
-             ((not (commit-id? str))
+             ((or (> (string-length str) 40)
+                  (not (string-every char-set:hex-digit str)))
               (resolve `(tag . ,str)))      ;definitely a tag
              (else
               (catch 'git-error