* checkout: Update returns UpdateResult so conflicts are visible

The internal 3-way merge already tracked which files were written, removed, and left with conflict markers, but Checkout.Update discarded all of it and returned a bare error. That collapsed a clean update and one that silently wrote <<<<<<< markers into working-tree files into the same outcome, with no way for a caller to tell them apart.

Update now returns (UpdateResult, error). UpdateResult.Conflicted lists paths that were merged but not cleanly -- this is still a successful update (err == nil), not an error, since the working tree is usable and the caller just needs to know which files to show a user. A genuine failure still returns a non-nil error paired with a zero-value UpdateResult. FilesWritten/FilesRemoved are populated the same way, as paths rather than counts.

Threading the paths up required changing the internal update's processFileUpdates from returning bare int counts to returning the actual []string paths, since the count was all that existed before. Checkout.Extract is untouched -- it has no conflict concept.

Breaking change, clean break at v0.7.0 (pre-1.0).

* checkout: sort Update result paths, document FilesWritten/Conflicted overlap

Update's result slices were built by ranging over maps.allNames, a map[string]bool, so their order was whatever Go's randomized map iteration happened to produce that run. A caller diffing result.FilesWritten against a golden file, or asserting on it with reflect.DeepEqual, would see it fail intermittently for no reason related to their code.

Sort all three slices (sort.Strings) right before Update returns. Added a test that touches four files -- two conflicting, two clean additions, named so map iteration order is very unlikely to hand them back sorted by luck -- and asserts the exact sorted slice on both FilesWritten and Conflicted. Verified it fails reliably (5/5 runs) with the sort calls temporarily removed, then restored the fix.

Also stated outright, in both the internal and public UpdateResult doc comments, what "clean or not" was only implying: every path in Conflicted also appears in FilesWritten, since the conflict markers are what got written there. check-in: 75f3521e54 user: [email protected] tags: trunk