Add more config IncludeIf conditions

[dscho]

Recently there was a mention on git-users to extend the includeIf statement to read config.

Now that seems like a nasty circular dependency but there's other low-hanging fruit there, like make it match a given env name to a value (or glob?).

Copied from: https://public-inbox.org/git/87in9ucsbb.fsf@evledraar.gmail.com/