Add bash shell completion command

[srfrog]

This PR adds a new completion command to the Dgraph main binary. It is used to generate bash and zsh shell completion code.

Bash example:

$ dgraph completion bash > dgraph.sh

# source in .bashrc
source dgraph.sh

---

This change is 

[golangcibot]

Error return value of RootCmd.GenBashCompletion is not checked (from errcheck)

[danielmai]

Tried it out. It seems pretty useful. Here's what it like it action (<TAB> means I pressed the tab key):

$ dgraph <TAB><TAB>
acl              bash-completion  cert             debug            live             version          
alpha            bulk             conv             increment        restore          zero             
$ dgraph alpha -<TAB><TAB>
--acl_access_ttl=      --badger.vlog=         --expand_edge          --log_dir=             -p                     --tls_dir=             --wal=
--acl_cache_ttl=       --bindall              --export=              --logtostderr          --pending_proposals=   --tls_use_system_ca    --whitelist=
--acl_refresh_ttl=     --block_rate=          --expose_trace         --lru_mb=              --port_offset=         --trace=               -z
--acl_secret_file=     --config=              --idx=                 --max_retries=         --postings=            -v                     --zero=
--alsologtostderr      --custom_tokenizers=   --jaeger.collector=    --mutations=           --profile_mode=        --v=                   
--auth_token=          --debugmode            -l                     --my=                  --query_edge_limit=    --vmodule=             
--badger.tables=       --enterprise_features  --log_backtrace_at=    -o                     --tls_client_auth=     -w       

[danielmai]

Zsh completion isn't working for me.

$ ./dgraph completion zsh
#compdef dgraph

_arguments \
  '1: :->level1' \
  '2: :->level2' \
  '3: :_files'
case $state in
  level1)
    case $words[1] in
      dgraph)
        _arguments '1: :(acl alpha bulk cert completion conv debug help increment live restore version zero)'
      ;;
      *)
        _arguments '*: :_files'
      ;;
    esac
  ;;
  level2)
    case $words[2] in
      acl)
        _arguments '2: :(chmod groupadd groupdel info passwd useradd userdel usermod)'
      ;;
      cert)
        _arguments '2: :(ls)'
      ;;
      completion)
        _arguments '2: :(bash zsh)'
      ;;
      *)
        _arguments '*: :_files'
      ;;
    esac
  ;;
  *)
    _arguments '*: :_files'
  ;;
esac
$ source <(./dgraph completion zsh)
_arguments:comparguments:319: can only be called from completion function
_arguments:comparguments:319: can only be called from completion function

[codexnull]

I agree that shell completion would be helpful but I'm not sure that it should be a core dgraph command. The output is fixed for a given dgraph release. The same could be accomplished by just adding static completion files to the release package.

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @srfrog)

[manishrjain]

This is useful. Either the user or our install scripts can use this to set up auto-completion based on whichever version dgraph is.

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @srfrog)

[srfrog]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @golangcibot)

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dgraph/cmd/root.go, line 167 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

Error return value of RootCmd.GenBashCompletion is not checked (from errcheck)

Done.