hgmerge: various cleanups Details: - put temporary file names into variables - make all temporary file names end with random part - cleanup FileMerge handling - do not use hardlinks in FileMerge change test (breaks on FAT) - try harder to keep file mtime unchanged in case of failed merge

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NONEXISTENT: No such file or directory
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NONEXISTENT: No such file in rev 9e1684505872
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