Make us be better at guessing a good hostname for the email.

It's still just a guess, and the result is not a real email
address anyway. If you want to, you can use COMMIT_AUTHOR_EMAIL
to correct for any git guesses.

commit-tree.c

@@ -317,8 +317,10 @@ int main(int argc, char **argv)
 	memcpy(realemail, pw->pw_name, len);
 	realemail[len] = '@';
 	gethostname(realemail+len+1, sizeof(realemail)-len-1);
+	if (!strchr(realemail+len+1, '.')) {
 		strcat(realemail, ".");
 		getdomainname(realemail+strlen(realemail), sizeof(realemail)-strlen(realemail)-1);
+	}
 	time(&now);
 	tm = localtime(&now);