Pass terminal dimensions to git by setting COLUMNS and LINES

This partially reverts "Show diff stats as wide as the terminal" (commit
a7eb3057ec70d4948ca5f2de98fcf29d844085b4) which passed the terminal
width to git-diff using `--stat=%(width)`. This feature was introduced
in Git version 1.7.10 released 2013-04-06 and causes problems for older
versions of Git.

It turns out that Git will automatically read $COLUMNS when showing diff
stats, so the same effect can be achieved by passing the terminal
dimensions using environment variables.

Fixes GH-151, "Broken commit rendering on Ubuntu 10.04" since
incompatibilitis with older versions of Git is no longer an issue.
Tested using `tig 1f384f3f5b9f0ea5a78504200eb577fcb8ba600a`.