pager: set COLUMNS to term_columns()

After we invoke the pager, our stdout goes to a pipe, not the
terminal, meaning we can no longer use an ioctl to get the
terminal width. For that reason, ad6c3739a3 (pager: find out
the terminal width before spawning the pager, 2012-02-12)
started caching the terminal width.

But that cache is only an in-process variable. Any programs
we spawn will also not be able to run that ioctl, but won't
have access to our cache. They'll end up falling back to our
80-column default.

You can see the problem with:

  git tag --column=row

Since git-tag spawns a pager these days, its spawned
git-column helper will see neither the terminal on stdout
nor a useful COLUMNS value (assuming you do not export it
from your shell already). And you'll end up with 80-column
output in the pager, regardless of your terminal size.

We can fix this by setting COLUMNS right before spawning the
pager. That fixes this case, as well as any more complicated
ones (e.g., a paged program spawns another script which then
generates columnized output).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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