Fix changelog parsing affecting caller timezone state

We meddle with TZ environ which then propagates to other values through
mktime() implicitly calling tzset(), but that other data doesn't get
reset by just restoring the TZ variable. Restore initial state by explicitly
call tzset() after we're done with it.

Fixes: #1821

build/parseChangelog.c

@@ -175,6 +175,7 @@ static int dateToTimet(const char * datestr, time_t * secs, int * date_words)
 	setenv("TZ", tz, 1);
 	free(tz);
     }
+    tzset();
 
     if (*secs == -1) goto exit;