This fix ignore some functions in the Go template when header is
redendered, so that `--format "{{truncate .ID 1}}"` will still
be able to redener the header correctly.
Additional test cases have been added to the unit test.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
This fix is part of the discussion in 28199 about using
`truncate` to replace `--no-trunc`.
As part of the fix, a new function `truncate` has been
added for Go templates so that it is possible to use
```
docker stack services --format "{{truncate .ID 5}}: {{.Mode}} {{.Replicas}}"
```
to show truncated ID.
A unit test has been added.
This fix is related to 28199.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
