sparse: replace merge action values with mergestate.ACTION_* constants Having bytestrings like `b'r'` makes it hard to understand for people who don't know the code much or looking at it for the first time. Differential Revision: https://phab.mercurial-scm.org/D8881

mergeresult: implement a len() function and use it In next patch we will start yielding from `getactions()` instead of building and returning a list. Hence we can no longer rely on that for getting us a length. Differential Revision: https://phab.mercurial-scm.org/D8880

merge: replace use of actions dict with mergeresult object There are still some places which can be improved by having a dedicated API, this patch for now make all users of actions dict go through the mergeresult object API. Differential Revision: https://phab.mercurial-scm.org/D8879

mergeresult: add sort argument to getactions() method This will be used in next patch. Differential Revision: https://phab.mercurial-scm.org/D8878

merge: pass mergeresult obj in merge._prefetchfiles() Differential Revision: https://phab.mercurial-scm.org/D8877

merge: pass mergeresult obj instead of actions in applyupdates() (API) This is similar to past 20 patches or so where we are replacing use of a bare actions dict with a dedicated mergeresult object. The goal is to have a dedicated powerful object instead of a dict while making the code more easier to understand. In past few patches, we have already simplified the code at some places using the newly introduced object. This patch does not updates applyupdates() to use the mergeresult object directly. That will be done in next patch to make things easier to review. Differential Revision: https://phab.mercurial-scm.org/D8876