Remove unnecessary call_depth parameter in merge-recursive.

Remove unnecessary call_depth parameter in merge-recursive.

Because the output_indent always matches the call_depth value
there is no reason to pass around the call_depth to the merge
function during each recursive invocation.

This is a simple refactoring that will make the code easier to
follow later on as I start to add output verbosity controls.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>