style.fish --all improvements

clang-format (since 10) can output diagnostics which indicate
lines needing formatting with --dry-run and -Werror: the exit
code indicates if a file is correctly formatted or not.

We used to copy each .cpp file, run clang_format on the duplicate
and then `cmp` to see if there were changes made, before just
printing a line with the filename and moving the new ontop of
the original.

Now we show clang-format diagnostics which indicate which
lines will be changed, prompt for confirmation and then let
clang-format modify the files in-place without the juggling.

Looks like this: https://user-images.githubusercontent.com/291142/184561633-c16754c8-179e-426b-ba15-345ba65b9cf9.png

build_tools/style.fish

@@ -74,13 +74,20 @@ if set -q c_files[1]
     else if type -q clang-format
         echo === Running "$red"clang-format"$normal"
         for file in $c_files
-            cp $file $file.new # preserves mode bits
+            if clang-format --dry-run -Werror $file
-            clang-format $file >$file.new
+                # file was clean, remove it from the list
-            if cmp --quiet $file $file.new
+                set -e c_files[(contains -i $file $c_files)]
-                rm $file.new
+            end
-            else
+        end
-                echo $file was NOT correctly formatted
+        if set -q c_files[1]
-                mv $file.new $file
+            printf "Reformat those %d files?\n" (count $c_files)
+            read -P 'y/N? ' -n1 -l ans
+            if string match -qi "y" -- $ans
+                clang-format -i --verbose $c_files
+            else if string match -qi "n" -- $ans
+                echo Skipping
+            else # like they ctrl-C'd or something.
+                exit 1
             end
         end
     else