git grep with leading inverted bracket expression

If the first atom of a regex is a bracket expression with an inverted range, git grep is very slow.

$ time git grep 'struct_size' >/dev/null

real 0m0.368s user 0m0.563s sys 0m0.453s

$ time git grep '[^t]truct_size' >/dev/null

real 0m31.529s user 1m54.909s sys 0m0.805s

If the bracket expression is moved to even the second position in the string, it runs much faster:

$ time git grep 's[^p]ruct_size' >/dev/null

real 0m3.989s user 0m13.939s sys 0m0.403s

It's pretty bad with even a '.' as the first character:

$ time git grep '.truct_size' >/dev/null

real 0m14.514s user 0m52.624s sys 0m0.598s

$ git --version git version 2.17.1

Setting LANG=C improves matters by a factor of 3-4 (depending if you count real or user time):

$ time git grep '[^t]truct_size' >/dev/null real 0m10.035s user 0m28.795s sys 0m0.537s

(this is using something pretty close to Linus' current HEAD of the linux repository, an i7-7500, 16GB memory).