commit-slab: clarify slabname##_peek()'s return value

Ever since 862e730ec1 (commit-slab: introduce slabname##_peek()
function, 2015-05-14) the slabname##_peek() function is documented as:

  This function is similar to indegree_at(), but it will return NULL
  until a call to indegree_at() was made for the commit.

This, however, is usually not the case.  If indegree_at() allocates
memory, then it will do so not only for the single commit it got as
parameter, but it will allocate a whole new, ~512kB slab.  Later on,
if any other commit's 'index' field happens to point into an already
allocated slab, then indegree_peek() for such a commit will return a
valid non-NULL pointer, pointing to a zero-initialized location in the
slab, even if no indegree_at() call has been made for that commit yet.

Update slabname##_peek()'s documentation to clarify this.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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