Merge branch 'hn/config-typo-advice' into seen

"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: suggest the correct form when key contains "=" in set context

diff --cc t/t1300-config.sh
Simple merge