git add: make -p/-i aware of sparse index

It is slow to expand a sparse index in-memory due to parsing of trees.
We aim to minimize that performance cost when possible. 'git add -p'
uses 'git apply' child processes to modify the index, but still there
are some expansions that occur.

It turns out that control flows out of cmd_add() in the interactive
cases before the lines that confirm that the builtin is integrated with
the sparse index.

Moving that integration point earlier in cmd_add() allows 'git add -i'
and 'git add -p' to operate without expanding a sparse index to a full
one.

Add test cases that confirm that these interactive add options work with
the sparse index.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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