debugindex etc.: add --changelog and --manifest options
These open the changelog and manifest, respectively, directly so you don't
need to specify the path.
The options have been added to debugindex, debugdata and debugrevlog.
The patch also fixes some minor usage-related bugs.
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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