tests/ci-commitmessage-submodules.sh: fix for RHCI
tests/ci-commitmessage-submodules.sh: fix for RHCI

Special-case when this script is run under RHCI, which will try to fetch
the merge commit if possible. Use RHCI_COMMIT instead to refer to the
actual PR/branch HEAD being evaluated.

Use realpath to workaround the developer's git dir being in a symbolic
link.

Closes: #857
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree