merge: Use `git_index__fill` to populate the indexvmg/index-fill

Instead of calling `git_index_add` in a loop, use the new
`git_index_fill` internal API to fill the index with the initial staged
entries.

The new `fill` helper assumes that all the entries will be unique and
valid, so it can append them at the end of the entries vector and only
sort it once at the end. It performs no validation checks.

This prevents the quadratic behavior caused by having to sort the
entries list once after every insertion.

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