hgweb: support disabling page cache
By default, hgweb_mod supports caching via the ETag header. This can
cause some confusion with browsers which cache aggressively. This change
preserves existing behavior while giving the administrator a knob to
disable the ETag header.
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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