scripts: update autocomplete per diff changes

per https://github.com/iterative/dvc/pull/3244#pullrequestreview-351108115

scripts/completion/dvc.bash

@@ -34,9 +34,10 @@ _dvc_install=''
 _dvc_lock='$(compgen -G *.dvc)'
 _dvc_metrics='add modify rmeove show'
 _dvc_metrics_add='-t --type -x --xpath $(compgen -G *)'
+_dvc_metrics_show='$(-t --type -x --xpath -a --all-branches -T --all-tags -R --recursive $(compgen -G *)'
+_dvc_metrics_diff='--targets -t --type -x --xpath -R --show-json'
 _dvc_metrics_modify='-t --type -x --xpath $(compgen -G *)'
 _dvc_metrics_remove='$(compgen -G *)'
-_dvc_metrics_show='$(-t --type -x --xpath -a --all-branches -T --all-tags -R --recursive $(compgen -G *)'
 _dvc_move='$(compgen -G *)'
 _dvc_pipeline='list show'
 _dvc_pipeline_list=''

scripts/completion/dvc.zsh

@@ -99,7 +99,8 @@ _dvc_destroy=(
 _dvc_diff=(
   "--show-json[Format the output into a JSON]"
   "--checksums[Display checksums for each entry]"
-  {1,2}":Git revision (e.g. branch, tag, SHA):"
+  "1:Git reference to the older version:"
+  "2:Git reference to the newer version:"
 )
 
 _dvc_fetch=(
@@ -160,7 +161,7 @@ _dvc_lock=(
 )
 
 _dvc_metrics=(
-  "1:Sub command:(show add modify remove)"
+  "1:Sub command:(add show diff modify remove)"
 )
 
 _dvc_move=(