git-download: Fix 'git-predicate' to use absolute paths.

git ls-files will return paths relative to the repository directory. This
commit prepends the repository directory to those paths when calling lstat,
such that 'git-predicate' works if the current working directory is not the
repository directory.

* guix/git-download.scm (git-predicate): Prepend repository directory to the
  file path when calling lstat.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/git-download.scm

@@ -146,7 +146,8 @@ absolute file name and STAT is the result of 'lstat'."
                           (line
                            (loop (cons line lines))))))
          (inodes      (map (lambda (file)
-                             (let ((stat (lstat file)))
+                             (let ((stat (lstat
+                                          (string-append directory "/" file))))
                                (cons (stat:dev stat) (stat:ino stat))))
                            files))
          (status      (close-pipe pipe)))