commit-graph: drop count_distinct_commits() function

When writing a commit graph, we collect a list of object ids in an array, which we'll eventually copy into an array of "struct commit" pointers. Before we do that, though, we count the number of distinct commit entries. There's a subtle bug in this step, though. We eliminate not only duplicate oids, but also in split mode, any oids which are not commits or which are already in a graph file. However, the loop starts at index 1, always counting index 0 as distinct. And indeed it can't be a duplicate, since we check for those by comparing against the previous entry, and there isn't one for index 0. But it could be a commit that's already in a graph file, and we'd overcount the number of commits by 1 in that case. That turns out not to be a problem, though. The only things we do with the count are: - check if our count will overflow our data structures. But the limit there is 2^31 commits, so while this is a useful check, the off-by-one is not likely to matter. - pre-allocate the array of commit pointers. But over-allocating by one isn't a problem; we'll just waste a few extra bytes. The bug would be easy enough to fix, but we can observe that neither of those steps is necessary. After building the actual commit array, we'll likewise check its count for overflow. So the extra check of the distinct commit count here is redundant. And likewise we use ALLOC_GROW() when building the commit array, so there's no need to preallocate it (it's possible that doing so is slightly more efficient, but if we care we can just optimistically allocate one slot for each oid; I didn't bother here). So count_distinct_commits() isn't doing anything useful. Let's just get rid of that step. Note that a side effect of the function was that we sorted the list of oids, which we do rely on in copy_oids_to_commits(), since it must also skip the duplicates. So we'll move the qsort there. I didn't copy the "TODO" about adding more progress meters. It's actually quite hard to make a repository large enough for this qsort would take an appreciable amount of time, so this doesn't seem like a useful note. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>