transaction: use posixfile and unlink from util
instead of open() and os.unlink()
Avoids potential issues with file access on Windows (e.g. AV-scanners).
test that 'hg commit' does not crash if the user removes a newly added file
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ touch b
$ hg add b
$ rm b
$ hg commit -A -m"comment #1"
removing b
nothing changed
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