Split out RelPath into a separate crate (#61029)

This is necessary to remove some `util` dependencies from crates, as
well as better sharing for our projects. This also includes the WIP
AbsPath abstraction as well as some bug fixes from internal tooling.


Release Notes:

- N/A or Added/Fixed/Improved ...

crates/git_ui/src/multi_diff_view.rs

@@ -94,7 +94,7 @@ fn register_entry(
             RelPath::new(rel, PathStyle::local())
                 .map(|r| r.into_owned().into())
                 .unwrap_or_else(|_| {
-                    RelPath::new(Path::new(MultiBuffer::DEFAULT_TITLE), PathStyle::Posix)
+                    RelPath::new(Path::new(MultiBuffer::DEFAULT_TITLE), PathStyle::Unix)
                         .unwrap()
                         .into_owned()
                         .into()
@@ -105,10 +105,10 @@ fn register_entry(
                 .new_path
                 .file_name()
                 .and_then(|n| n.to_str())
-                .and_then(|s| RelPath::new(Path::new(s), PathStyle::Posix).ok())
+                .and_then(|s| RelPath::new(Path::new(s), PathStyle::Unix).ok())
                 .map(|r| r.into_owned().into())
                 .unwrap_or_else(|| {
-                    RelPath::new(Path::new(MultiBuffer::DEFAULT_TITLE), PathStyle::Posix)
+                    RelPath::new(Path::new(MultiBuffer::DEFAULT_TITLE), PathStyle::Unix)
                         .unwrap()
                         .into_owned()
                         .into()