Add missing environment variable for --skip-unchanged

Fixes #581

CHANGELOG.md

@@ -10,6 +10,9 @@ Added the option `--strip-cr`. This removes all carriage return
 characters before diffing, which is helpful when dealing with a mix of
 Windows and non-Windows flies.
 
+The option `--skip-unchanged` now has a corresponding environment
+option `DFT_SKIP_UNCHANGED`.
+
 ## 0.52 (released 8th October 2023)
 
 ### Parsing

src/options.rs

@@ -218,6 +218,7 @@ json: Output the results as a machine-readable JSON array with an element per fi
         )
         .arg(
             Arg::new("skip-unchanged").long("skip-unchanged")
+                .env("DFT_SKIP_UNCHANGED")
                 .help("Don't display anything if a file is unchanged.")
         )
         .arg(