completely solve catKey memory leak

Since 4aaa584eb632a981f5364c844f9293d4cdedaa65 was incomplete, not being
able to get the right mode of the file when the index differs from HEAD,
this is a final workaround. Only buffering the start of the file
in this case avoids leaking memory.

This does not prevent git-cat-file being asked to output the whole file,
which needs to be consumed, and can be slow. But this only happens in a
rare edge case.

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