sha1_name: expose grab_nth_branch_switch()

wt-status.c:grab_1st_switch() contains a re-implementation of the
parsing logic in grab_nth_branch_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 is set in
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 the field.

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