Don't trust the manifest's computed branch for revision locked manifests.
Don't trust the manifest's computed branch for revision locked manifests.

The approach is correct- we should defer to the target manifest rather
than the originating patch- but we don't have the necessary info for
revision locked manifests to do so correctly.

BUG=None
TEST=unittests
TEST=cbuildbot --remote --lkgm -g 28805 mario-paladin --test-bootstrap

Change-Id: Ic2321527a7f538897bde281d8508dc31fd5ed973
Reviewed-on: https://gerrit.chromium.org/gerrit/29389
Commit-Ready: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite,zhang0137/chromite