Subzero: Add "make format-diff" target.

Makefile.standalone

 # The following variables will likely need to be modified, depending on where
 # and how you built LLVM & Clang. They can be overridden in a command-line
 # invocation of make, like:
 #
 #   make LLVM_SRC_PATH=<path> LLVM_BIN_PATH=<path> ...
 #
 
 # LLVM_SRC_PATH is the path to the root of the checked out source code. This
 # directory should contain the configure script, the include/ and lib/
 # directories of LLVM, Clang in tools/clang/, etc.
@@ skipping 58 matching lines @@
 
 build:
         @mkdir -p $@
 
 check: llvm2ice
         LLVM_BIN_PATH=$(LLVM_BIN_PATH) \
         $(LLVM_SRC_PATH)/utils/lit/lit.py -sv tests_lit
         (cd crosstest; LLVM_BIN_PATH=$(LLVM_BIN_PATH) ./runtests.sh)
 
 # TODO: Fix the use of wildcards.
+# Assumes clang-format is within $PATH.
 format:
-        $(LLVM_BIN_PATH)/clang-format -style=LLVM -i \
-        src/Ice*.h src/Ice*.cpp src/llvm2ice.cpp
+        clang-format -style=LLVM -i src/*.h src/*.cpp
+
+# Assumes clang-format-diff.py is within $PATH, and that the
+# clang-format it calls is also within $PATH.  This may require adding
+# a component to $PATH, or creating symlinks within some existing
+# $PATH component.  Uses the one in /usr/lib/clang-format/ if it
+# exists.
+ifeq (,$(wildcard /usr/lib/clang-format/clang-format-diff.py))
+  CLANG_FORMAT_DIFF = clang-format-diff.py
+else
+  CLANG_FORMAT_DIFF = /usr/lib/clang-format/clang-format-diff.py
+endif
+format-diff:
+        git diff -U0 HEAD^ | \
+        $(CLANG_FORMAT_DIFF) -p1 -style=LLVM -i
 
 clean:
         rm -rf llvm2ice *.o build/