t7800: simplify basic usage test

Use "test_line_count" instead of "wc -l", use "git -C" instead of a
subshell, and use test_expect_code when calling difftool.  Ease
debugging by capturing output into temporary files.

Suggested-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7800-difftool.sh

@@ -24,16 +24,15 @@ prompt_given ()
 }
 
 test_expect_success 'basic usage requires no repo' '
-	lines=$(git difftool -h | grep ^usage: | wc -l) &&
+	test_expect_code 129 git difftool -h >output &&
-	test "$lines" -eq 1 &&
+	grep ^usage: output &&
 	# create a ceiling directory to prevent Git from finding a repo
 	mkdir -p not/repo &&
-	ceiling="$PWD/not" &&
+	test_when_finished rm -r not &&
-	lines=$(cd not/repo &&
+	test_expect_code 129 \
-		GIT_CEILING_DIRECTORIES="$ceiling" git difftool -h |
+	env GIT_CEILING_DIRECTORIES="$(pwd)/not" \
-		grep ^usage: | wc -l) &&
+	git -C not/repo difftool -h >output &&
-	test "$lines" -eq 1 &&
+	grep ^usage: output
-	rmdir -p not/repo
 '
 
 # Create a file on master and change it on branch