Fix permissions always shown as 'changed' with git-difftool

CHANGELOG.md

@@ -8,6 +8,9 @@ Added support for Smali.
 
 ### Display
 
+Fixed an issue where all files would show a permissions change when
+using difftastic with `git difftool`.
+
 Fixed an issue with paths not showing the containing directory when
 using difftastic with `git difftool`.
 

src/options.rs

@@ -321,6 +321,14 @@ impl FileArgument {
     pub(crate) fn permissions(&self) -> Option<FilePermissions> {
         match self {
             FileArgument::NamedPath(path) => {
+                // When used with `git difftool`, the first argument
+                // is a temporary file that always has the same
+                // permissions. That doesn't mean the file permissions
+                // have changed, so we shouldn't compare.
+                if is_git_tmpfile(path) {
+                    return None;
+                }
+
                 let metadata = std::fs::metadata(path).ok()?;
                 Some(metadata.permissions().into())
             }