column: show auto columns when pager is active

When columns are set to automatic for git tag and the output is
paginated by git, the output is a single column instead of multiple
columns.

Standard behaviour in git is to honor auto values when the pager is
active, which happens for example with commands like git log showing
colors when being paged.

Since ff1e72483 (tag: change default of `pager.tag` to "on",
2017-08-02), the pager has been enabled by default, exposing this
problem to more people.

finalize_colopts in column.c only checks whether the output is a TTY to
determine if columns should be enabled with columns set to auto. Also
check if the pager is active.

Adding a test for git column is possible but requires some care to work
around a race on stdin. See commit 18d8c2693 (test_terminal: redirect
child process' stdin to a pty, 2015-08-04). Test git tag instead, since
that does not involve stdin, and since that was the original motivation
for this patch.

Helped-by: Rafael Ascensão <rafa.almas@gmail.com>
Signed-off-by: Kevin Daudt <me@ikke.info>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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