config: parse more robust format in GIT_CONFIG_PARAMETERS

config: parse more robust format in GIT_CONFIG_PARAMETERS

When we stuff config options into GIT_CONFIG_PARAMETERS, we shell-quote
each one as a single unit, like:

  'section.one=value1' 'section.two=value2'

On the reading side, we de-quote to get the individual strings, and then
parse them by splitting on the first "=" we find. This format is
ambiguous, because an "=" may appear in a subsection. So the config
represented in a file by both:

  [section "subsection=with=equals"]
  key = value

and:

  [section]
  subsection = with=equals.key=value

ends up in this flattened format like:

  'section.subsection=with=equals.key=value'

and we can't tell which was desired. We have traditionally resolved this
by taking the first "=" we see starting from the left, meaning that we
allowed arbitrary content in the value, but not in the subsection.

Let's make our environment format a bit more robust by separately
quoting the key and value. That turns those examples into:

  'section.subsection=with=equals.key'='value'

and:

  'section.subsection'='with=equals.key=value'

respectively, and we can tell the difference between them. We can detect
which format is in use for any given element of the list based on the
presence of the unquoted "=". That means we can continue to allow the
old format to work to support any callers which manually used the old
format, and we can even intermingle the two formats. The old format
wasn't documented, and nobody was supposed to be using it. But it's
likely that such callers exist in the wild, so it's nice if we can avoid
breaking them. Likewise, it may be possible to trigger an older version
of "git -c" that runs a script that calls into a newer version of "git
-c"; that new version would see the intermingled format.

This does create one complication, which is that the obvious format in
the new scheme for

  [section]
  some-bool

is:

  'section.some-bool'

with no equals. We'd mistake that for an old-style variable. And it even
has the same meaning in the old style, but:

  [section "with=equals"]
  some-bool

does not. It would be:

  'section.with=equals=some-bool'

which we'd take to mean:

  [section]
  with = equals=some-bool

in the old, ambiguous style. Likewise, we can't use:

  'section.some-bool'=''

because that's ambiguous with an actual empty string. Instead, we'll
again use the shell-quoting to give us a hint, and use:

  'section.some-bool'=

to show that we have no value.

Note that this commit just expands the reading side. We'll start writing
the new format via "git -c" in a future patch. In the meantime, the
existing "git -c" tests will make sure we didn't break reading the old
format. But we'll also add some explicit coverage of the two formats to
make sure we continue to handle the old one after we move the writing
side over.

And one final note: since we're now using the shell-quoting as a
semantically meaningful hint, this closes the door to us ever allowing
arbitrary shell quoting, like:

  'a'shell'would'be'ok'with'this'.key=value

But we have never supported that (only what sq_quote() would produce),
and we are probably better off keeping things simple, robust, and
backwards-compatible, than trying to make it easier for humans. We'll
continue not to advertise the format of the variable to users, and
instead keep "git -c" as the recommended mechanism for setting config
(even if we are trying to be kind not to break users who may be relying
on the current undocumented format).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>