Loop invariant code slows down bisection when AST verification fails

[akaihola]

As noted by @rogalski in #205,

run_black, diff_and_get_opcodes and opcodes_to_chunks are run in each pass of bisect loop, despite being independent from context_lines variable:

darker/src/darker/__main__.py

Line 150 in d269159

formatted = run_black(rev2_isorted, black_config)

darker/src/darker/__main__.py

Line 155 in d269159

opcodes = diff_and_get_opcodes(rev2_isorted, formatted)

darker/src/darker/__main__.py

Line 158 in d269159

black_chunks = list(opcodes_to_chunks(opcodes, rev2_isorted, formatted))

These lines should be moved to be executed before the loop.

If AST verification fails, Darker starts to bisect in order to find the smallest possible number of extra diff context lines which preserves the AST. Moving loop invariant code outside the bisection loop will speed up this process.