largefiles: put whole "hgmerge" process into the same "wlock" scope Before this patch, there are two distinct "wlock" scopes below in "hgmerge": 1. "merge.update" via original "hg.merge" function 2. "updatelfiles" specific "wlock" scope (to synchronize largefile dirstate) But these should be executed in the same "wlock" scope for consistency, because users of "hg.merge" don't get "wlock" explicitly before invocation of it. - merge in commands This patch puts almost all of the original "hgmerge" implementation into "_hgmerge" to reduce changes.