Fix sha1-truncation, by removing it.

Usually, the "%h" format should result in an abbreviated sha1, but it
looks like getting it through `git archive' returns the full thing
rather than the truncated version.  So avoid the substring for now, and
find out if it's a git bug or not.
(cherry picked from commit af6364a58c)

collects/repo-time-stamp/stamp.rkt

@@ -35,7 +35,7 @@
       (let* ([x (x)]
              [m (and (string? x) (regexp-match rx:secs+id x))]
              [d (and m (seconds->date (string->number (cadr m))))]
-             [id (and m (substring (caddr m) 0 10))]
+             [id (and m (caddr m))]
              [how (and m (cadddr m))])
         (define (pad02 f) (let ([n (f d)]) (if (< n 10) (format "0~a" n) n)))
         (and d (format "~a-~a-~a(~a/~a)"