cvsserver: Use Merged response instead of Update-existing for merged files

cvsserver: Use Merged response instead of Update-existing for merged files

Using Update-existing leads to the client forgetting about the "locally
modified" status of the file which can lead to loss of local changes on
later updates.

Signed-off-by: Frank Lichtenheld <redacted>
Acked-by: Martin Langhoff <redacted>
Signed-off-by: Junio C Hamano <redacted>