Pass terminal dimensions to git by setting COLUMNS and LINES

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`.