merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/contrib/credential/libsecret/Makefile

@@ -0,0 +1,25 @@
+MAIN:=git-credential-libsecret
+all:: $(MAIN)
+
+CC = gcc
+RM = rm -f
+CFLAGS = -g -O2 -Wall
+PKG_CONFIG = pkg-config
+
+-include ../../../config.mak.autogen
+-include ../../../config.mak
+
+INCS:=$(shell $(PKG_CONFIG) --cflags libsecret-1 glib-2.0)
+LIBS:=$(shell $(PKG_CONFIG) --libs libsecret-1 glib-2.0)
+
+SRCS:=$(MAIN).c
+OBJS:=$(SRCS:.c=.o)
+
+%.o: %.c
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(INCS) -o $@ -c $<
+
+$(MAIN): $(OBJS)
+	$(CC) -o $@ $(LDFLAGS) $^ $(LIBS)
+
+clean:
+	@$(RM) $(MAIN) $(OBJS)