Only set LINES and COLUMNS environment variables for external commands

Setting the LINES/COLUMNS environment variables to a constant number
breaks ncurses' getmaxyx() logic, so dynamic resizing doesn't work as
pointed out by Marcel Partap in a comment to the fix for issue #151
(https://github.com/jonas/tig/commit/f0ad1f74#commitcomment-3528715).

As a work-around, only set LINES/COLUMNS when forking Git processes. This
way diff stats will still use the whole terminal width and resizing works
as intended.

Fixes #109