Bug using `fetch` with blank `-c` arguments to git

[Adam Dinwoodie <adam@dinwoodie.org>]

While investigating some issues with a different project, I discovered
the command `git -c config.helper= fetch` was working with the Debian
stable version of Git (v2.30.2) but not with my local build
(v2.34.1.428.gdcc0cd074f).

Specifically, I see the following output:

$ ./git -c credential.helper= fetch
error: bogus format in GIT_CONFIG_PARAMETERS
fatal: unable to parse command-line config

Investigating with `git bisect`, the change in behaviour seems to have
been introduced in 1ff21c05ba ("config: store "git -c" variables using
more robust format", 2021-01-12).

I see the same behaviour with `-c config.helper=`, `-c
core.autocrlf=`, `-c core.autocrlf` and `-c core.autocrlf=true`..
Notably the behaviour does not affect all other git commands; `git -c
core.autocrlf= log -1` works as expected.

I think this is a regression; I can't see any reason why these
commands shouldn't work.

Curiously, I'm seeing this behaviour on both my Raspberry Pi OS and
Debian Bullseye systems, but not my Cygwin systems. I've not yet tried
to work out what the difference is there. In all cases, I was testing
with my own build, built with `make -j<num> configure && ./configure
--prefix=$HOME/.local && make -j<num>`.