etcupdate: remove redundant diff3 flag

[thj]

Summary

With diff3 -A and -m select different and conflicting output modes. When
both are passed -m is used by gnu diff3. Remove the extra -A flag.

Test Plan

The reduntant behaviour can be demonstrated by:

$ cd /usr/src/usr.bin/diff3/tests/
$ diff3 -m -A -L 1.txt -L 2.txt -L 3.txt 1.txt 2.txt 3.txt  > one
$ diff3 -m -L 1.txt -L 2.txt -L 3.txt 1.txt 2.txt 3.txt  > two   
$ diff one two

[pstef]

I don't use etcupdate yet, but this change looks reasonable to me.

[0mp]

I run your test case and it seems like -A can safely be removed.

Note: I think there is something not right with the grammar in the summary. Otherwise, it seems to be ready to be committed. An MFC would be nice as well.