Make RequiredPrefixForAccel() "see through" capturing groups.

Fixes #289.

Change-Id: Id0c888e3cbe898f8c6f7858b8bde05f38c787b73
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58131
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed