Add make style-modified

As it runs fourmolu only on modified files, it's a lot faster.

CONTRIBUTING.md

@@ -145,6 +145,7 @@ Code Style
 
 We use automated formatting with Fourmolu to enforce a unified style across the code bases. It is checked in the CI process.
 After installing Fourmolu ≥ 0.12, you can automatically format the code bases with `make style` at the top level of the project.
+You can also use `make style-modified` to only format modified files.
 
 Other Conventions
 -----------------

Makefile

@@ -27,6 +27,15 @@ style: ## Run the code styler
 		! -path Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs \
 		| xargs -P $(PROCS) -I {} fourmolu -q -i {}
 
+style-modified: ## Run the code styler on modified files
+	@git ls-files --modified Cabal Cabal-syntax cabal-install \
+		-X Cabal-syntax/src/Distribution/Fields/Lexer.hs \
+		-X Cabal-syntax/src/Distribution/SPDX/LicenseExceptionId.hs \
+		-X Cabal-syntax/src/Distribution/SPDX/LicenseId.hs \
+		-X Cabal/src/Distribution/Simple/Build/Macros/Z.hs \
+		-X Cabal/src/Distribution/Simple/Build/PathsModule/Z.hs \
+		| grep '.hs$$' | xargs -P $(PROCS) -I {} fourmolu -q -i {}
+
 # source generation: Lexer
 
 LEXER_HS:=Cabal-syntax/src/Distribution/Fields/Lexer.hs