Make config reading continue after hitting a missing include file.

For example, if you have

[include]
path = foo

and foo didn't exist, git_config_open_ondisk() would just give up
on the rest of the file.  Now it ignores the unresolved include
without error and continues reading the rest of the file.
We should only be continuing if the error is that the file wasn't found (`GIT_ENOTFOUND`). Writing it like this would ignore other OS errors or OOM.