filemerge: use a single temp dir instead of temp files This can help to remove the clutter from UIs that display just the filenames; instead of seeing foo~local.C9ru9r.txt and foo~base.2DMV22.txt (in the /tmp directory on most platforms), we create a single new directory and use that, producing filenames like /tmp/hgmerge.C9ru9r/foo~local.txt. Differential Revision: https://phab.mercurial-scm.org/D2888