apply-one-time-sed.sh: modernize style

Convert `[ ... ]` to use `test` and test for the existence of a regular
file (`-f`) instead of any file (`-e`).

Move the `then`s onto their own lines so that it conforms with the
general test style.

Instead of redirecting input into sed, allow it to open its own input.

Use `cmp -s` instead of `diff` since we only care about whether the two
files are equal and `diff` is overkill for this.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-httpd/apply-one-time-sed.sh

@@ -7,11 +7,13 @@
 #
 # This can be used to simulate the effects of the repository changing in
 # between HTTP request-response pairs.
-if [ -e one-time-sed ]; then
+if test -f one-time-sed
+then
 	"$GIT_EXEC_PATH/git-http-backend" >out
-	sed "$(cat one-time-sed)" <out >out_modified
+	sed "$(cat one-time-sed)" out >out_modified
 
-	if diff out out_modified >/dev/null; then
+	if cmp -s out out_modified
+	then
 		cat out
 	else
 		cat out_modified