filter-branch: remove an unnecessary use of 'git read-tree'

The intent of this particular call to 'git read-tree' was to fill an index. But in fact, it only allocated an empty index. Later in the program, the index is filled anyway by calling read-tree with specific commits, and considering that elsewhere the index is even removed (i.e., it is not relied upon that the index file exists), this first call of read-tree is completely redundant. Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>