Convert Changeset timestamp to UTC before saving

This fixes display of the commit time in the wrong timezone when the original commit received from source control contains timezone info as was the case with SourceGitea plugin. Fixes#419 (originally reported in PR #416)

Author: dregad

Source/Source.API.php

@@ -954,7 +954,10 @@ function save() {
 # Commit timestamp: can't use DATE_ATOM format to insert datetime,
 # as MySQL < 8.0.19 does not support specifying timezone
 # @see https://dev.mysql.com/doc/refman/8.0/en/datetime.html
-$t_timestamp = $this->timestamp->format( 'Y-m-d H:i:s' );
+# so we convert the timestamp to UTC first.
+$t_timestamp = $this->timestamp
+ ->setTimezone( newDateTimeZone( 'UTC' ) )
+ ->format( 'Y-m-d H:i:s' );
 
 if ( 0 == $this->id ) { # create
 $t_query = "INSERT INTO $t_changeset_table ( repo_id, revision, parent, branch, user_id,