Command line completion updates for "git -c var.name=val"
* sg/complete-configuration-variables:
completion: complete config variables and values for 'git clone --config='
completion: complete config variables names and values for 'git clone -c'
completion: complete values of configuration variables after 'git -c var='
completion: complete configuration sections and variable names for 'git -c'
completion: split _git_config()
completion: simplify inner 'case' pattern in __gitcomp()
completion: use 'sort -u' to deduplicate config variable names
completion: deduplicate configuration sections
completion: add tests for 'git config' completion
completion: complete more values of more 'color.*' configuration variables
completion: fix a typo in a comment