* Error-handling is cleaned up to only let a file-not-found error
through, not other sorts of errors. And when a file-not-found
error happens, we clean up the error.
* Test now checks that file-not-found introduces no error. And
other minor cleanups.
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 a number of tests that modify the global or system
search paths during the tests. This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.
