mergetools: vimdiff: silence annoying messages

mergetools: vimdiff: silence annoying messages

When using the single window mode we are greeted with the following
warning:

  "./content_LOCAL_8975" 6L, 28B
  "./content_BASE_8975" 6 lines, 29 bytes
  "./content_REMOTE_8975" 6 lines, 29 bytes
  "content" 16 lines, 115 bytes
  Press ENTER or type command to continue

every time.

Silence that.

Suggested-by: Fernando Ramos <redacted>
Signed-off-by: Felipe Contreras <redacted>
Reviewed-by: Fernando Ramos <redacted>
Signed-off-by: Junio C Hamano <redacted>