merge: fix documented order of `labels` argument Differential Revision: https://phab.mercurial-scm.org/D12150

filemerge: when not keeping premerge, don't write markers to context When premerge is enabled (as it is for non-binary inputs by default) and the markers are not kept, we currently still write it to the output context and then restore the previous content right after. With the refactoring in the previous patch, we can easily avoid that step and instead write the output in the opposite case (i.e. when it's successful or when the markers are supposed to be kept). Differential Revision: https://phab.mercurial-scm.org/D12149