global: convert trivial usages of test <expr> -a/-o <expr>

Our coding guidelines say to not use `test` with `-a` and `-o` because
it can easily lead to bugs. Convert trivial cases where we still use
these to instead instead concatenate multiple invocations of `test` via
`&&` and `||`, respectively.

While not all of the converted instances can cause ambiguity, it is
worth getting rid of all of them regardless:

    - It becomes easier to reason about the code as we do not have to
      argue why one use of `-a`/`-o` is okay while another one isn't.

    - We don't encourage people to use these expressions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -489,13 +489,13 @@ find_existing_splits () {
 			;;
 		END)
 			debug "Main is: '$main'"
-			if test -z "$main" -a -n "$sub"
+			if test -z "$main" && test -n "$sub"
 			then
 				# squash commits refer to a subtree
 				debug "  Squash: $sq from $sub"
 				cache_set "$sq" "$sub"
 			fi
-			if test -n "$main" -a -n "$sub"
+			if test -n "$main" && test -n "$sub"
 			then
 				debug "  Prior: $main -> $sub"
 				cache_set $main $sub