Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets

Add targets to compile the various *.o files we declared in commonly
used *_OBJS variables. This is useful for debugging purposes, to
e.g. get to the point where we can compile a git.o. See [1] for a
use-case for this target.

https://lore.kernel.org/git/YBCGtd9if0qtuQxx@coredump.intra.peff.net/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -669,6 +669,8 @@ ETAGS_TARGET = TAGS
 FUZZ_OBJS += fuzz-commit-graph.o
 FUZZ_OBJS += fuzz-pack-headers.o
 FUZZ_OBJS += fuzz-pack-idx.o
+.PHONY: fuzz-objs
+fuzz-objs: $(FUZZ_OBJS)
 
 # Always build fuzz objects even if not testing, to prevent bit-rot.
 all:: $(FUZZ_OBJS)
@@ -686,6 +688,8 @@ PROGRAM_OBJS += http-backend.o
 PROGRAM_OBJS += imap-send.o
 PROGRAM_OBJS += sh-i18n--envsubst.o
 PROGRAM_OBJS += shell.o
+.PHONY: program-objs
+program-objs: $(PROGRAM_OBJS)
 
 # Binary suffix, set to .exe for Windows builds
 X =
@@ -2386,13 +2390,19 @@ XDIFF_OBJS += xdiff/xmerge.o
 XDIFF_OBJS += xdiff/xpatience.o
 XDIFF_OBJS += xdiff/xprepare.o
 XDIFF_OBJS += xdiff/xutils.o
+.PHONY: xdiff-objs
+xdiff-objs: $(XDIFF_OBJS)
 
 TEST_OBJS := $(patsubst %$X,%.o,$(TEST_PROGRAMS)) $(patsubst %,t/helper/%,$(TEST_BUILTINS_OBJS))
+.PHONY: test-objs
+test-objs: $(TEST_OBJS)
 
 GIT_OBJS += $(LIB_OBJS)
 GIT_OBJS += $(BUILTIN_OBJS)
 GIT_OBJS += common-main.o
 GIT_OBJS += git.o
+.PHONY: git-objs
+git-objs: $(GIT_OBJS)
 
 OBJECTS += $(GIT_OBJS)
 OBJECTS += $(PROGRAM_OBJS)
@@ -2402,6 +2412,8 @@ OBJECTS += $(FUZZ_OBJS)
 ifndef NO_CURL
 	OBJECTS += http.o http-walker.o remote-curl.o
 endif
+.PHONY: objects
+objects: $(OBJECTS)
 
 dep_files := $(foreach f,$(OBJECTS),$(dir $f).depend/$(notdir $f).d)
 dep_dirs := $(addsuffix .depend,$(sort $(dir $(OBJECTS))))