test-pathconflicts-merge: stop requiring symlink support The errors from the last time I took a shot at this back in early November have disappeared, so let's just enable this.

commands: use constants for merge things We have nice constants now. Let's use them to make the code easier to reason about. Differential Revision: https://phab.mercurial-scm.org/D2702

merge: use constants for actions We finish up establishing named constants in this file with actions. I remember scratching my head trying to figure out what this code was doing as part of addressing a recent security issue with subrepos. Having the named constants in place definitely makes things easier to read. I'm not convinced the new constants have the best names (I'm not an expert in this code). But they can be changed easily enough. Also, since these constants are internal only, we might want to change their values to something more human readable to facilitate debugging. Or maybe we could employ an enum type some day... Differential Revision: https://phab.mercurial-scm.org/D2701