config: improve diagnostic for "set" with missing value

config: improve diagnostic for "set" with missing value

"git config set pull.rebase=false" currently fails with "wrong
number of arguments", and the implicit form "git config
pull.rebase=false" fails with "invalid key". Neither points at
the real problem: the value is missing.

Report that directly, and when the argument has the shape
"<valid-key>=<value>", also suggest the split form:

    $ git config set pull.rebase=false
    error: missing value to set to the variable 'pull.rebase=false'
    hint: did you mean "git config set pull.rebase false"?

When the prefix before "=" is not a valid key, drop the hint:

    $ git config set foo=bar
    error: missing value to set to a variable with an invalid name 'foo=bar'

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>