[bug] generic issue with git_config handlers

  One of my co-workers stumbled upon a misfeature of the git config
parser. The following syntax is allowed:

    [section]
	 foo

  I saw that this is a feature, though as a consequence, the "value"
passed to git_config handlers may be NULL, and a _lot_ of git config
handlers don't know this could happen. This becomes an issue when you do
something like:

[user]
    name

--> every git command segfaults basically

[alias]
    foo

--> `git foo` segfaults

  I wanted to fix that, and generate nicer errors than a crash, changing
git_config to also take a boolean argument telling if the caller expects
"value" to be NULL, or would like to reject it, though the code has so
many callbacks to fix, and I have too little time right now, that I just
drop the thing on the list, hoping that some nice soul will take care of
the issue.

Cheers,