sha1dc: build git plumbing code more explicitly

The plumbing code between sha1dc and git is defined in
sha1dc_git.[ch], but these aren't compiled / included directly but
only via the indirect inclusion from sha1dc code.  This is slightly
confusing when you try to trace the build flow.

This patch brings the following changes for simplification:

  - Make sha1dc_git.c stand-alone and build from Makefile

  - sha1dc_git.h is the common header to include further sha1.h
    depending on the build condition

  - Move comments for plumbing codes from the header to definitions

This is also meant as a preliminary work for further plumbing with
external sha1dc shlib.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1472,6 +1472,8 @@ ifdef APPLE_COMMON_CRYPTO
 	BASIC_CFLAGS += -DSHA1_APPLE
 else
 	DC_SHA1 := YesPlease
+	BASIC_CFLAGS += -DSHA1_DC
+	LIB_OBJS += sha1dc_git.o
 ifdef DC_SHA1_SUBMODULE
 	LIB_OBJS += sha1collisiondetection/lib/sha1.o
 	LIB_OBJS += sha1collisiondetection/lib/ubc_check.o
@@ -1481,12 +1483,9 @@ else
 	LIB_OBJS += sha1dc/ubc_check.o
 endif
 	BASIC_CFLAGS += \
-		-DSHA1_DC \
 		-DSHA1DC_NO_STANDARD_INCLUDES \
 		-DSHA1DC_INIT_SAFE_HASH_DEFAULT=0 \
 		-DSHA1DC_CUSTOM_INCLUDE_SHA1_C="\"cache.h\"" \
-		-DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C="\"sha1dc_git.c\"" \
-		-DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H="\"sha1dc_git.h\"" \
 		-DSHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C="\"git-compat-util.h\""
 endif
 endif

hash.h

@@ -8,11 +8,7 @@
 #elif defined(SHA1_OPENSSL)
 #include <openssl/sha.h>
 #elif defined(SHA1_DC)
-#ifdef DC_SHA1_SUBMODULE
-#include "sha1collisiondetection/lib/sha1.h"
-#else
-#include "sha1dc/sha1.h"
-#endif
+#include "sha1dc_git.h"
 #else /* SHA1_BLK */
 #include "block-sha1/sha1.h"
 #endif

sha1dc_git.c

@@ -1,8 +1,8 @@
-/*
- * This code is included at the end of sha1dc/sha1.c with the
- * SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C macro.
- */
+#include "cache.h"
 
+/*
+ * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
+ */
 void git_SHA1DCFinal(unsigned char hash[20], SHA1_CTX *ctx)
 {
 	if (!SHA1DCFinal(hash, ctx))
@@ -11,6 +11,9 @@ void git_SHA1DCFinal(unsigned char hash[20], SHA1_CTX *ctx)
 	    sha1_to_hex(hash));
 }
 
+/*
+ * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
+ */
 void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *vdata, unsigned long len)
 {
 	const char *data = vdata;

sha1dc_git.h

@@ -1,16 +1,12 @@
-/*
- * This code is included at the end of sha1dc/sha1.h with the
- * SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H macro.
- */
+/* Plumbing with collition-detecting SHA1 code */
+
+#ifdef DC_SHA1_SUBMODULE
+#include "sha1collisiondetection/lib/sha1.h"
+#else
+#include "sha1dc/sha1.h"
+#endif
 
-/*
- * Same as SHA1DCFinal, but convert collision attack case into a verbose die().
- */
 void git_SHA1DCFinal(unsigned char [20], SHA1_CTX *);
-
-/*
- * Same as SHA1DCUpdate, but adjust types to match git's usual interface.
- */
 void git_SHA1DCUpdate(SHA1_CTX *ctx, const void *data, unsigned long len);
 
 #define platform_SHA_CTX SHA1_CTX