merge.applyupdates: only attempt to merge files in mergeactions This only makes a difference when a merge driver is active -- in that case we don't want to try and merge all the files, just the ones still unresolved after the merge driver's preprocess step is over.

filemerge: rename _symlinkcheck to _mergecheck We're going to be adding other checks in here soon.

test-merge-prompt.t: rename to test-merge-changedelete.t Because: (a) the only prompts we test in this file are change/delete prompts, and (b) we're going to be expanding the scope of this test to cover future improvements to change/delete conflicts, including avoiding prompts altogether.