fix merge-tree where two branches share no changes
Here we fix a regression which was introduced by
15b4f7a68d8c3c8ee28424415b203f61202d65d1 /
merge-tree: use ll_merge() not xdl_merge()
Which caused merge-tree to segfault in particular combinations of
merging files which existed in one branch, but not in the other or in
the merge-base. This was caused by referencing entry->path at a time
when entry was known to be possibly-NULL.
To correct the problem, we save the path of the entry we came in with,
as the path should be the same among all the stages no matter which
sides are involved in the merge.