index: always return at least the root for filter prefixes

index: always return at least the root for filter prefixes

Return at least an entry for the prefix itself for each prefix passed
to filter.  Otherwise something like "bup save ... x/y" will fail when
x is up to date because filter will return nothing, save will traverse
nothing in its main loop, and bup will crash with an assertion
failure:

  File "/home/rlb/src/bup/main/cmd/bup-save", line 440, in <module>
    assert(len(shalists) == 1)

Instead "bup save ... x/y" should (and now will) produce a new save
containing x/y.

Thanks to Simon Persson for helping formulate a good test case.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>