t4014: do not use "slave branch" nomenclature

Git branches have been qualified as topic branches, integration branches,
development branches, feature branches, release branches and so on.
Git has a branch that is the master *for* development, but it is not
the master *of* any "slave branch": Git does not have slave branches,
and has never had, except for a single testcase that claims otherwise. :)

Independent of any future change to the naming of the "master" branch,
removing this sole appearance of the term is a strict improvement: it
avoids divisive language, and talking about "feature branch" clarifies
which developer workflow the test is trying to emulate.

Reported-by: Till Maas <tmaas@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4014-format-patch.sh

@@ -81,16 +81,16 @@ test_expect_success 'format-patch --ignore-if-in-upstream handles tags' '
 '
 
 test_expect_success "format-patch doesn't consider merge commits" '
-	git checkout -b slave master &&
+	git checkout -b feature master &&
 	echo "Another line" >>file &&
 	test_tick &&
-	git commit -am "Slave change #1" &&
+	git commit -am "Feature branch change #1" &&
 	echo "Yet another line" >>file &&
 	test_tick &&
-	git commit -am "Slave change #2" &&
+	git commit -am "Feature branch change #2" &&
 	git checkout -b merger master &&
 	test_tick &&
-	git merge --no-ff slave &&
+	git merge --no-ff feature &&
 	git format-patch -3 --stdout >patch &&
 	grep "^From " patch >from &&
 	test_line_count = 3 from