merge-recursive: get rid of the index_only global variable
merge-recursive: get rid of the index_only global variable

struct merge_options already has a call_depth member, and index_only
global variable always equals to !!call_depth.

We always use index_only as a condition, so we can just
use call_depth instead of index_only.

Signed-off-by: Miklos Vajna <16f13092e03e3d9e997de904d0f86cc803fb6ef7@frugalware.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git