Check that a revision has been provided before trying to parse it

When setting up Tekuti for the first time the ‘compute-bogus-comments’ will get
the current commit passed in and try to get the parent. Since there is only the
“initial commit” at this stage it will get ‘#f’ as the parent commit and pass it
in to ‘fold-commits’. Trying to parse revision ‘#f’ will surely fail.

The ‘fold-commits’ function already expects that ‘rev’ might be empty, so this
should work out fine.

tekuti/git.scm

@@ -334,7 +334,7 @@
                    (cons (string->symbol k) v))))))
 
 (define (fold-commits f rev seed)
-  (let lp ((rev (git-rev-parse rev)) (seed seed))
+  (let lp ((rev (and rev (git-rev-parse rev))) (seed seed))
     (if rev
         (let ((commit (parse-commit rev)))
           (lp (assq-ref commit 'parent)