Teach format-patch to handle output directory relative to cwd

Without any explicit -o parameter, we correctly avoided putting the
resulting patch output to the toplevel.  We should do the same when
the user gave a relative pathname to be consistent with this case.

Noticed by Cesar Eduardo Barros.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4014-format-patch.sh

@@ -3,7 +3,7 @@
 # Copyright (c) 2006 Junio C Hamano
 #
 
-test_description='Format-patch skipping already incorporated patches'
+test_description='various format-patch tests'
 
 . ./test-lib.sh
 
@@ -230,4 +230,54 @@ test_expect_success 'shortlog of cover-letter wraps overly-long onelines' '
 
 '
 
+test_expect_success 'format-patch from a subdirectory (1)' '
+	filename=$(
+		rm -rf sub &&
+		mkdir -p sub/dir &&
+		cd sub/dir &&
+		git format-patch -1
+	) &&
+	case "$filename" in
+	0*)
+		;; # ok
+	*)
+		echo "Oops? $filename"
+		false
+		;;
+	esac &&
+	test -f "$filename"
+'
+
+test_expect_success 'format-patch from a subdirectory (2)' '
+	filename=$(
+		rm -rf sub &&
+		mkdir -p sub/dir &&
+		cd sub/dir &&
+		git format-patch -1 -o ..
+	) &&
+	case "$filename" in
+	../0*)
+		;; # ok
+	*)
+		echo "Oops? $filename"
+		false
+		;;
+	esac &&
+	basename=$(expr "$filename" : ".*/\(.*\)") &&
+	test -f "sub/$basename"
+'
+
+test_expect_success 'format-patch from a subdirectory (3)' '
+	here="$TEST_DIRECTORY/$test" &&
+	rm -f 0* &&
+	filename=$(
+		rm -rf sub &&
+		mkdir -p sub/dir &&
+		cd sub/dir &&
+		git format-patch -1 -o "$here"
+	) &&
+	basename=$(expr "$filename" : ".*/\(.*\)") &&
+	test -f "$basename"
+'
+
 test_done