Teach git the 'commit-graph' builtin that will be used for writing and
reading packed graph files. The current implementation is mostly
empty, except for an '--object-dir' option.

Since 'git commit-graph' is a builtin command, it shouldn't show up in
completion when doing 'git co<TAB>'.
Please squash in the patch below to make it so.

Furthermore, please have a look at
  
  https://public-inbox.org/git/20180202160132.31550-1-szeder.dev@gmail.com/

for an other oneliner change.

diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 17929b0809..fafed13c06 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash

@@ -841,6 +841,7 @@ __git_list_porcelain_commands ()
 		check-ref-format) : plumbing;;
 		checkout-index)   : plumbing;;
 		column)           : internal helper;;
+		commit-graph)     : plumbing;;
 		commit-tree)      : plumbing;;
 		count-objects)    : infrequent;;
 		credential)       : credentials;;