Callers of `git_config__cvar` already handle the case where the
function returns an error due to a failed configuration variable
lookup, but we are actually swallowing errors when calling
`git_config__lookup_entry` inside of the function.

Fix this by returning early when `git_config__lookup_entry`
returns an error. As we call `git_config__lookup_entry` with
`no_errors == false` which leads us to call `get_entry` with
`GET_NO_MISSING` we will not return early when the lookup fails
due to a missing entry. Like this we are still able to set the
default value of the cvar and exit successfully.