log-tree.c: small refactor in show_signature()

log-tree.c: small refactor in show_signature()

The next patch needs to show the result of signature verification on a
mergetag extended header in a way similar to how embedded signature for
the commit object itself is shown. Separate out the logic to go through
the message lines and show them in the "error" color (highlighted) or the
"correct" color (dim).

Signed-off-by: Junio C Hamano <redacted>