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.
Sure.  I was trying to figure out the conventions in the file which weren't entirely clear to me (the test immediately before it unlinks its files, but others don't).  I'll update this soon.