* 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.

We have been refreshing on read and write for a while now, so
git_config_refresh() is at best a no-op, and might just end up wasting
cycles.

In the check for multiline, we traverse the backslashes from the end
backwards and int the end assert that we haven't gone past the beginning
of the line. We make sure of this in the loop condition, but we also
check in the return value.

However, for certain configurations, a line in a multiline variable
might be empty to aid formatting. In that case, 'end' == 'start', since
we ended up looking at the first char which made it a multiline.

There is no need for the (end > start) check in the return, since the
loop guarantees we won't go further back than the first char in the
line, and we do accept the first char to be the final backslash.

This fixes #2483.