tag-git: add a colon after the "Changes since" header

The original is Markdown, but in the announcement mail and in the tag
message, we do not use Markdown, so the fact that this line is a header
is lost. Let's append a colon at the end of that line to make the
message flow a bit easier.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

update-scripts/tag-git.sh

@@ -85,6 +85,7 @@ else
 	raw_notes="$(sed -n "/^## Changes since/,\${:1;p;n;/^## Changes/q;b1}" \
 			<"$build_extra_dir"/ReleaseNotes.md)" &&
 	notes="$(echo "$raw_notes" |
+		sed '1s/^## Changes since.*/&:/' |
 		markdown |
 		LC_CTYPE=C w3m -dump -cols 72 -T text/html)" &&
 	tag_message="$(printf "%s\n\n%s" \