icasefs: enhance test to prevent regression by changes in the future This patch adds steps to test case-folding collision detection on the revision tree built up in the order different from the one reproducing issue3452. This may prevent regression by changes around "copy detection" and/or "case-folding collision detection" logic in the future.

icasefs: rewrite case-folding collision detection (issue3452) Before this patch, case-folding collision detection uses "copies.pathcopies()" before "manifestmerge()", and is not aware of renaming in some cases. For example, in the case of issue3452, "copies.pathcopies()" can't detect renaming, if the file is renamed at the revision before common ancestor of merging. So, "hg merge" is aborted unexpectedly on case insensitive filesystem. This patch fully rewrites case-folding collision detection, and relocate it into "manifestmerge()". New implementation uses list of actions held in "actions" and "prompts" to build provisional merged manifest up. Provisional merged manifest should be correct, if actions required to build merge result up in working directory are listed up in "actions" and "prompts" correctly. This patch checks case-folding collision still before prompting for merge, to avoid aborting after some interactions with users. So, this assumes that user would choose not "deleted" but "changed". This patch also changes existing abort message, because sorting before collision detection changes order of checked files.

icasefs: refactor "test-casecollision-merge.t" to reuse in succeeding fixes This patch refactors "test-casecollision-merge.t" to increase reusability in succeeding patches. It is confirmed that changed test also can detect issue3370.

icasefs: ignore removed files at building "dirstate._foldmap" up on icasefs Before this patch, all files in dirstate are used to build "_foldmap" up on case insensitive filesystem regardless of their statuses. For example, when dirstate contains both removed file 'a' and added file 'A', "_foldmap" may be updated finally by removed file 'a'. This causes unexpected status information for added file 'A' at "hg status" invocation. This patch ignores removed files at building "_foldmap" up on case insensitive filessytem. This patch doesn't add any test, because this issue is difficult to reproduce intentionally: it depends on iteration order of "dirstate._map".