sha1_name: expose grab_nth_branch_switch()

wt-status re-implements it almost exactly in grab_1st_switch().  As a
first step towards cleaning up this duplication, expose
grab_nth_branch_switch() and the struct grab_nth_branch_switch_cbdata
via cache.h.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
wt-status: remove unused field in grab_1st_switch_cbdata

The struct grab_1st_switch_cbdata has the field found, which in set by
grab_1st_switch() when a match is found.  This information is redundant
and unused by any caller: the return value of the function serves to
communicate this information anyway.  Remove it.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>