tg--awksome.sh: -tm= for topmsg_header and correct comments

tg--awksome.sh: -tm= for topmsg_header and correct comments

Support passing the same -tm= option to run_awk_topmsg_header
that run_awk_topgit_msg supports except that the leading
<branch_name>: part is optional in this case.  Now the utility
function v_get_tmopt can easily be used with run_awk_topmsg_header.

While in there, also clean up the incorrect and outdated comments.

The comments for run_awk_topgit_msg have apparently always been
wrong about the output and some other comments were rendered incorrect
by the recent change to automatically supply the empty blob.

Fix all of these up.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>