[ref #152] no silent failure if path conversion isn't possible

For now we are loud about it for the lack of a better way to do certain
transformations on windows.

The API remains the same though, assuming that eventually we will
do some rewriting and possibly return a PathBuf instead, hence 'Cow'.

git-ref/src/mutable.rs

@@ -52,7 +52,7 @@ impl FullName {
 
     /// Convert this name into the relative path, lossily, identifying the reference location relative to a repository
     pub fn to_path(&self) -> Cow<'_, Path> {
-        self.0.to_path_lossy()
+        self.0.to_path().expect("UTF-8 conversion always succeeds").into()
     }
 
     /// Dissolve this instance and return the buffer.

git-ref/src/name.rs

@@ -8,7 +8,7 @@ pub type Error = git_validate::reference::name::Error;
 impl<'a> FullName<'a> {
     /// Convert this name into the relative path identifying the reference location.
     pub fn to_path(self) -> Cow<'a, Path> {
-        self.0.to_path_lossy()
+        self.0.to_path().expect("UTF-8 conversion always succeeds").into()
     }
 
     /// Return ourselves as byte string which is a valid refname
@@ -21,7 +21,7 @@ impl<'a> PartialName<'a> {
     /// Convert this name into the relative path possibly identifying the reference location.
     /// Note that it may be only a partial path though.
     pub fn to_partial_path(self) -> Cow<'a, Path> {
-        self.0.to_path_lossy()
+        self.0.to_path().expect("UTF-8 conversion always succeeds").into()
     }
 
     /// Provide the name as binary string which is known to be a valid partial ref name.