repo: standardize help behavior
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

GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,lipro-yocto/git-repo,lewixliu/git-repo,codetutil/git-repo,lipro-yocto/git-repo,GerritCodeReview/git-repo,aosp-mirror/tools_repo,aosp-mirror/tools_repo,gabbayo/git-repo,gabbayo/git-repo,mer-tools/git-repo,couchbasedeps/git-repo,TheQtCompany/git-repo,mer-tools/git-repo,TheQtCompany/git-repo,codetutil/git-repo