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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# HG changeset patch
# User #author#
# Date #date|date#
# Node ID #node#
#parent%changesetparent#
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