cat-file: add remove_timestamp helper

maybe_remove_timestamp() takes arguments, but it would be useful to have
a function that reads from stdin and strips the timestamp. This would
allow tests to pipe data into a function to remove timestamps, and
wouldn't have to always assign a variable. This is especially helpful
when the data is multiple lines.

Keep maybe_remove_timestamp() the same, but add a remove_timestamp
helper that reads from stdin.

The tests in the next patch will make use of this.

Signed-off-by: John Cai <johncai86@gmail.com>

t/t1006-cat-file.sh

@@ -105,13 +105,18 @@ strlen () {
 }
 
 maybe_remove_timestamp () {
-    if test -z "$2"; then
-        echo_without_newline "$1"
-    else
-	echo_without_newline "$(printf '%s\n' "$1" | sed -e 's/ [0-9][0-9]* [-+][0-9][0-9][0-9][0-9]$//')"
-    fi
+	if test -z "$2"; then
+		echo_without_newline "$1"
+	else
+		echo_without_newline "$(printf '%s\n' "$1" | remove_timestamp)"
+	fi
 }
 
+remove_timestamp () {
+	sed -e 's/ [0-9][0-9]* [-+][0-9][0-9][0-9][0-9]$//'
+}
+
+
 run_tests () {
     type=$1
     sha1=$2