When writing out, parse the resulting file instead of adding or
replacing the value locally. This has the effect of reading external
changes as well.

On set, we set/add the value written to the config's internal values,
but we do not refresh old values.

Document this in a test in preparation for the refresh changes.

This adds giterr_user_cancel to return GIT_EUSER and clear any
error message that is sitting around.  As a result of using that
in places, we need to be more thorough with capturing errors that
happen inside a callback when used internally.  To help with that,
this also adds giterr_capture and giterr_restore so that when we
internally use a foreach-type function that clears errors and
converts them to GIT_EUSER, it is easier to restore not just the
return value, but the actual error message text.