core.precomposeunicode with decomposed filenames

Commit 750b2e4785e shows a failure of core.precomposeunicode
when decomposed filenames are in the index.

The problem when decomposed file names are in the index and readdir()
converts them into the decomposed form.
"Git status" will report the precomposed for as untracked.

Solution:
Precompose file names when reading the index file from disc into memory.