hg-download: Use invoke instead of system*.

* guix/build/hg.scm (hg-fetch): Use invoke and remove vestigial plumbing.

guix/build/hg.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2018 Mark H Weaver <mhw@netris.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -33,19 +34,20 @@
   "Fetch CHANGESET from URL into DIRECTORY.  CHANGESET must be a valid
 Mercurial changeset identifier.  Return #t on success, #f otherwise."
 
-  (and (zero? (system* hg-command
+  (invoke hg-command
           "clone" url
           "--rev" changeset
           ;; Disable TLS certificate verification.  The hash of
           ;; the checkout is known in advance anyway.
           "--insecure"
-                       directory))
-       (with-directory-excursion directory
-         (begin
+          directory)
+
   ;; The contents of '.hg' vary as a function of the current
   ;; status of the Mercurial repo.  Since we want a fixed
   ;; output, this directory needs to be taken out.
-           (delete-file-recursively ".hg")
-           #t))))
+  (with-directory-excursion directory
+    (delete-file-recursively ".hg"))
+
+  #t)
 
 ;;; hg.scm ends here