xdiff/xhistogram: pass arguments directly to fall_back_to_classic_diff

xdiff/xhistogram: pass arguments directly to fall_back_to_classic_diff

By passing the 'xpp' and 'env' argument directly to the function
'fall_back_to_classic_diff', we eliminate an occurrence of the 'index'
in histogram_diff, which will prove useful in a bit.

While at it, move it up in the file. This will make the diff of
one of the next patches more legible.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>