Control the maximum count permitted by `GlobalReplace()`.

Change-Id: I0e6f72951c0b6fa8d1c42021c646022cc1a2a14c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60810
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Perry Lorier <perryl@google.com>