Merge branch 'hn/config-typo-advice'

"git config foo.bar=baz" is not likely to be a request to read the value of such a variable with '=' in its name; rather it is plausible that the user meant "git config set foo.bar baz". Give advice when giving an error message. * hn/config-typo-advice: config: improve diagnostic for "set" with missing value config: add git_config_key_is_valid() for quiet validation