Update pre-commit hook to check scala formatting

This commit updates the pre-commit hook to run scalafmt the same way it
already runs prettier. Apart from the exit code tweak, this basically
involved copying the pre-commit hook from
guardian/datawrapper-import#12

As with the prettier part of the hook, this should only run the
formatter on files with staged changes that don’t have unstaged changes,
and for any with both that fail formatting it should just report an
error and exit.

scripts/git-hooks/pre-commit

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+hookExitCode=0
+
 if command -v fronts-client/node_modules/.bin/prettier > /dev/null; then
     echo "Checking file formatting with prettier…"
     echo ""
@@ -23,7 +25,6 @@ if command -v fronts-client/node_modules/.bin/prettier > /dev/null; then
         | xargs -0 fronts-client/node_modules/.bin/prettier --list-different)
     if test -z "$prettierFormattedOutput" -a -z "$prettierCheckedOutput" ; then
         echo "All good"
-        exit 0
     else
         if test -n "$prettierFormattedOutput" ; then
             echo "The following staged files needed formatting by prettier! Formatting has been
@@ -46,8 +47,63 @@ overwriting unstaged changes. Please format them yourself or bypass this hook wi
             echo "$prettierCheckedOutput"
             echo ""
         fi
-        exit 1
+        hookExitCode=1
     fi
 else
     echo "Note: fronts-client/node_modules/.bin/prettier not found, so skipping formatting check"
 fi
+
+if command -v scalafmt > /dev/null; then
+    echo "Checking file formatting with scalafmt…"
+    echo ""
+    # For files which have staged changes and no unstaged changes, let scalafmt format them
+    scalaFilesWithOnlyStagedChanges=$(
+        comm -1 -3 \
+            <(git diff --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+            <(git diff --cached --name-only --diff-filter=ACMR | grep "\.scala\$" | sort))
+    _scalafmtFormattedOutput=$(
+        comm -1 -3 \
+            <(git diff --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+            <(git diff --cached --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+        | tr "\n" "\0" \
+        | xargs -0 scalafmt --reportError --no-stderr --debug)
+    scalafmtFormattedExit=$?
+    # For files which have both staged and unstaged changes, check them with scalafmt but don’t modify them
+    scalafmtCheckedOutput=$(
+        comm -1 -2 \
+            <(git diff --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+            <(git diff --cached --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+        | tr "\n" "\0" \
+        | xargs -0 scalafmt --list --reportError --no-stderr)
+    scalafmtCheckedExit=$?
+    if test "$scalafmtFormattedExit" -eq 0 -a "$scalafmtCheckedExit" -eq 0 ; then
+        echo "All good"
+    else
+        if test "$scalafmtFormattedExit" -ne 0; then
+            echo "The following staged files needed formatting by scalafmt! Formatting has been
+applied but the files have not been re-staged: please verify the changes, stage,
+and commit again."
+            echo ""
+            scalaFilesWhichHaveBeenFormatted=$(
+                comm -1 -2 \
+                    <(git diff --name-only --diff-filter=ACMR | grep "\.scala\$" | sort) \
+                    <(echo "$scalaFilesWithOnlyStagedChanges"))
+            echo "$scalaFilesWhichHaveBeenFormatted"
+            echo ""
+        fi
+        if test "$scalafmtCheckedExit" -ne 0; then
+            echo "The following staged files with additional unstaged changes were reported by
+scalafmt as needing formatting: these files have not been formatted, to avoid
+clobbering unstaged changes. Please format them yourself or bypass this hook with
+--no-verify."
+            echo ""
+            echo "$scalafmtCheckedOutput"
+            echo ""
+        fi
+        hookExitCode=1
+    fi
+else
+    echo "Note: scalafmt not found, so skipping formatting check"
+fi
+
+exit $hookExitCode