Merge branch 'hn/config-typo-advice'

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