git_index_find() in index_insert() is useless if replace is not
requested (append). Do not call it in this case.
It speedup git_index_append() *dramatically* on large indexes.

$ cat index_test.c

int main(int argc, char **argv)
{
        git_index *index;
        git_repository *repo;
        git_odb *odb;
        struct git_index_entry entry;
        git_oid tree_oid;
        char tree_hex[41];
        int i;

        git_repository_init(&repo, "/tmp/myrepo", 0);
        odb = git_repository_database(repo);
        git_repository_index(&index, repo);

        memset(&entry, 0, sizeof(entry));
        git_odb_write(&entry.oid, odb, "", 0, GIT_OBJ_BLOB);
        entry.path = "test.file";

        for (i = 0; i < 50000; i++)
                git_index_append2(index, &entry);

        git_tree_create_fromindex(&tree_oid, index);
        git_oid_fmt(tree_hex, &tree_oid);
        tree_hex[40] = '\0';
        printf("tree: %s\n", tree_hex);

        git_index_free(index);
        git_repository_free(repo);

        return 0;
}

Before:
$ time ./index_test
tree: 43f73659c43b651588cc81459d9e25b08721b95d
./index_test  151.19s user 0.05s system 99% cpu 2:31.78 total

After:
$ time ./index_test
tree: 43f73659c43b651588cc81459d9e25b08721b95d
./index_test  0.05s user 0.00s system 94% cpu 0.059 total

About 2573 times speedup on this test :)

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>