gnu: emacs-undercover: Don't use unstable tarball.

* gnu/packages/emacs-xyz.scm (emacs-undercover)[source]: Use GIT-FETCH and
GIT-FILE-NAME.

gnu/packages/emacs-xyz.scm

@@ -2333,13 +2333,13 @@ (define-public emacs-undercover
     (version "0.6.0")
     (source
      (origin
-       (method url-fetch)
-       (uri (string-append "https://github.com/sviridov/undercover.el/"
-                           "archive/v" version ".tar.gz"))
-       (file-name (string-append name "-" version ".tar.gz"))
+       (method git-fetch)
+       (uri (git-reference
+             (url "https://github.com/sviridov/undercover.el.git")
+             (commit (string-append "v" version))))
+       (file-name (git-file-name name version))
        (sha256
-        (base32
-         "0f48fi0xnbsqs382rgh85m9mq1wdnr0yib7as9xhwzvq0hsr5m0a"))))
+        (base32 "06qcvbp5rd0kh3ibrxj5p6r578lwsrgd7yj5c6slwmkdmna2fj33"))))
     (build-system emacs-build-system)
     (propagated-inputs
      `(("emacs-dash" ,emacs-dash)