repo: standardize help behavior

Standard utilities exit normally/zero when users explicitly request
--help, and they write to stdout.  Exiting non-zero & using stderr
is meant for incorrect tool usage instead.  We're already doing this
for `repo help <init|gitc-init>` calls, so lets fix `repo help` and
`repo --help|-h` to match.

Change-Id: Ia4f352b431c91eefef70dcafc11f00209ee69809

repo

@@ -776,8 +776,8 @@ The most commonly used repo commands are:
       """  help      Display detailed help on a command
 
 For access to the full online help, install repo ("repo init").
-""", file=sys.stderr)
-  sys.exit(1)
+""")
+  sys.exit(0)
 
 
 def _Help(args):