cros_merge_to_branch: tighten up help output
cros_merge_to_branch: tighten up help output

No functional changes.

BUG=None
TEST=`cros_merge_to_branch -h` looks nice

Change-Id: Idf2109b91033d957f4c907fb661e7de015eeb09c
Reviewed-on: https://gerrit.chromium.org/gerrit/46743
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite