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.
There are some files named "included" but those are the ones mentioned in the `include.path` directory, rather than files which include others.