MFC r264480: mergemaster: Avoid "/var/tmp/temproot disappeared" if there is

MFC r264480: mergemaster: Avoid "/var/tmp/temproot disappeared" if there is
nothing to compare.

Because of the change to find in SVN r253886, the entire temproot would be
deleted if it became empty, leading to a confusing message "*** FATAL ERROR:
The temproot directory ${TEMPROOT} has disappeared!"

Note that mergemaster does not do anything useful in this situation anyway
(e.g. put IGNORE_FILES="/etc/group /etc/master.passwd" in
/etc/mergemaster.rc and run mergemaster -p).

As noted in that commit, add -mindepth 1.

PR:		bin/188485
Submitted by:	David Boyd

git-svn-id: svn://svn.freebsd.org/base/stable/10@266953 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f