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
1:3aa14bbc23d9
0:8633637036c1
3aa14bbc23d9 tip
8633637036c1
8633637036c1+
8633637036c1
3aa14bbc23d9 tip
merging file1
3aa14bbc23d9 tip
3aa14bbc23d9 tip
3aa14bbc23d9 tip
3aa14bbc23d9 tip