Update presubmit branch check text for git 1.8

Update presubmit branch check text for git 1.8
The old option recommendation is deprecated.

R=jvoung@chromium.org
BUG=

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-llvm.git;a=commit;h=64dafdc

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-llvm.git;a=commit;h=618534a

[jvoung - send to chromium..., 2013-01-25 21:51:09]

lgtm