builtin-merge.c: allocate correct amount of memory

builtin-merge.c: allocate correct amount of memory

Fix two memory allocation errors which allocate space for a pointer
rather than enough space for the structure itself.

This:

    struct commit_list *parent = xmalloc(sizeof(struct commit_list *));

should have been this:

    struct commit_list *parent = xmalloc(sizeof(struct commit_list));

But while we're at it, change the allocation to reference the
variable it is allocating memory for to try to prevent a similar
mistake, for example if the type is changed, in the future.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Acked-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>