Merge branch 'jc/maint-abbrev-option-cli'

We did not document that many commands take unique prefix
abbreviations of long options (e.g. "--option" may be the only flag
that the command accepts that begin with "--opt", in which case you
can give "--opt") anywhere easy to find for new people.

* jc/maint-abbrev-option-cli:
  gitcli: describe abbreviation of long options