dot/gitconfig.in: Define a plausible word-diff regexp.

dot/gitconfig.in: Define a plausible word-diff regexp.

Match sequences of alphanumerics and loose non-space punctuation.  This
seems to give relatively good results on plain text with colour output.
Picking out differences in code might be harder, but line-based diffs
work better in such cases anyway: word diff is useful precisely because
it works well on reflowed text, and that's more likely to happen in a
text document than in code.