Option to avoid rewriting existing grid-wrap imports

[eli-schwartz]

When running isort on a codebase that already has imports formatted like this:

from foobar import (
    abc, bcd, cde, dff, efg, fgh, ghi, hij, ijk,
    jkl, klm, lmn, mno, nop, opq, pqr, qrs, rst,
    stu, tuv, uvw, vwx, wxy, xyz, yza, zab,
)

And the latest commit removes the use of "abc" so the import is also removed, isort wants to "fix" my imports so it reshuffles all 3 lines. This is very noisy for commit logs, and I'd rather permit the first line to go shorter and stay that way. I'll probably eventually add another import back, and the line will grow again... I don't care if it is balanced.

I think what I probably want is that isort should apply whichever change it would otherwise apply, but only if something was actually sorted (or optionally, if the max line length was exceeded), not if the only change would be to rebalance it.