`git2::RevparseMode::is_no_single` has a misleading name

[dkareh]

git2-rs/src/lib.rs

Line 617 in da6c126

is_bit_set!(is_no_single, RevparseMode::SINGLE);

That function returns true when the SINGLE bit is set, but its name implies the opposite. It seems like a typo from the original pull request.