tag: respect pager.tag in list-mode only

Using, e.g., `git -c pager.tag tag -a new-tag` results in errors such as
"Vim: Warning: Output is not to a terminal" and a garbled terminal.
Someone who makes use of both `git tag -a` and `git tag -l` will
probably not set `pager.tag`, so that `git tag -a` will actually work,
at the cost of not paging output of `git tag -l`.

Use the mechanisms introduced in two earlier patches to ignore
`pager.tag` in git.c and let the `git tag` builtin handle it on its own.
Only respect `pager.tag` when running in list-mode.

There is a window between where the pager is started before and after
this patch. This means that early errors can behave slightly different
before and after this patch. Since operation-parsing has to happen
inside this window, this can be seen with `git -c pager.tag="echo pager
is used" tag -l --unknown-option`. This change in paging-behavior should
be acceptable since it only affects erroneous usages.

Update the documentation and update tests.

If an alias is used to run `git tag -a`, then `pager.tag` will still be
respected. Document this known breakage. It will be fixed in a later
commit. Add a similar test for `-l`, which works.

Noticed-by: Anatoly Borodin <anatoly.borodin@gmail.com>
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -461,6 +461,9 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 			cmdmode = 'l';
 	}
 
+	if (cmdmode == 'l')
+		setup_auto_pager("tag", 0);
+
 	if ((create_tag_object || force) && (cmdmode != 0))
 		usage_with_options(git_tag_usage, options);