largefiles: check existence of the file with case awareness of the filesystem Before this patch, largefiles extension always unlinks largefiles untracked on the target context in merging/updating after updating working directory. For example, it is assumed that the revision X consists of ".hglf/A" (and "A" implicitly) and revision Y consists of "a" (not ".hglf/A"). In the case of updating from X to Y, largefiles extension tries to unlink "A" after updating "a" in working directory. This causes unexpected unlinking "a" on the case insensitive filesystem. This patch checks existence of the file in the working context with case awareness of the filesystem to prevent from such unexpected unlinking. "lfcommands._updatelfile()" also unlinks target file in the case "largefile is tracked in the target context, but fails to be fetched". This patch doesn't apply "repo.dirstate.normalize()" in this case, because it should be already ensured in the manifest merging that there is no normal file colliding against any largefiles.