testing/glab: add shell completion

- remove the article 'an' or 'a' as it is redundant
- add missing '# Contributor:' field for the current maintainer
- create shell completion subpackagees for bash, zsh and fish
7 jobs for !16094 with glab in 2 minutes and 9 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #278365
docker-alpine x86_64
lint

00:00:19

 
  Build
passed #278370
docker-alpine ci-build aarch64
build-aarch64

00:01:01

passed #278371
docker-alpine ci-build armv7
build-armv7

00:01:33

passed #278369
docker-alpine ci-build ppc64le
build-ppc64le

00:01:42

passed #278368
docker-alpine ci-build s390x
build-s390x

00:01:48

passed #278367
docker-alpine ci-build x86
build-x86

00:00:50

passed #278366
docker-alpine ci-build x86_64
build-x86_64

00:00:50