Normalize \ to / in diff output on Windows (fixes #3240)

CHANGELOG.md

@@ -12,6 +12,7 @@
   re-enabled with `--enable=useless-use-of-cat` or equivalent directive.
 - SC2015 about `A && B || C` no longer triggers when B is a test command.
 - SC3012: Do not warn about `\<` and `\>` in test/[] as specified in POSIX.1-2024
+- Diff output now uses / as path separator on Windows
 ### Fixed
 - SC2218 about function use-before-define is now more accurate.
 - SC2317 about unreachable commands is now less spammy for nested ones.

src/ShellCheck/Formatter/Diff.hs

@@ -191,11 +191,17 @@ splitLast x =
     let (last, rest) = splitAt 1 $ reverse x
     in (reverse rest, last)
 
+-- git patch does not like `\` on Windows
+normalizePath path =
+    case path of
+        c:rest -> (if c == pathSeparator then '/' else c) : normalizePath rest
+        [] -> []
+
 formatDoc color (DiffDoc name lf regions) =
     let (most, last) = splitLast regions
     in
-          (color bold $ "--- " ++ ("a" </> name)) ++ "\n" ++
+          (color bold $ "--- " ++ (normalizePath $ "a" </> name)) ++ "\n" ++
-          (color bold $ "+++ " ++ ("b" </> name)) ++ "\n" ++
+          (color bold $ "+++ " ++ (normalizePath $ "b" </> name)) ++ "\n" ++
           concatMap (formatRegion color LinefeedOk) most ++
           concatMap (formatRegion color lf) last