Not determine the shell from `.sh` extension

See discussion on issue #1369 for details.

src/ShellCheck/Analytics.hs

@@ -484,7 +484,7 @@ checkShebang params (T_Annotation _ list t) =
 checkShebang params (T_Script id sb _) = execWriter $ do
     unless (shellTypeSpecified params) $ do
         when (sb == "") $
-            err id 2148 "Tips depend on target shell and yours is unknown. Add a shebang or an extension to the filename."
+            err id 2148 "Tips depend on target shell and yours is unknown. Add a shebang or a .bash, .ksh, .dash extension to the filename."
         when (executableFromShebang sb == "ash") $
             warn id 2187 "Ash scripts will be checked as Dash. Add '# shellcheck shell=dash' to silence."
     unless (null sb) $ do

src/ShellCheck/Checker.hs

@@ -51,9 +51,10 @@ tokenToPosition startMap t = fromMaybe fail $ do
 shellFromFilename filename = foldl mplus Nothing candidates
   where
     shellExtensions = [(".ksh", Ksh)
-                      ,(".sh", Sh)
                       ,(".bash", Bash)
                       ,(".dash", Dash)]
+                      -- The `.sh` is too generic to determine the shell:
+                      -- We fallback to Bash in this case and emit SC2148 if there is no shebang
     candidates =
         map (\(ext,sh) -> if ext `isSuffixOf` filename then Just sh else Nothing) shellExtensions