git-pack-objects: write the pack files with a SHA1 csum

We want to be able to check their integrity later, and putting the
sha1-sum of the contents at the end is a good thing.  The writing
routines are generic, so we could try to re-use them for the index file,
instead of having the same logic duplicated.

Update unpack-objects to know about the extra 20 bytes at the end
of the index.

csum-file.h

@@ -0,0 +1,17 @@
+#ifndef CSUM_FILE_H
+#define CSUM_FILE_H
+
+/* A SHA1-protected file */
+struct sha1file {
+	int fd, error;
+	unsigned long offset;
+	SHA_CTX ctx;
+	unsigned char buffer[8192];
+};
+
+extern struct sha1file *sha1create(const char *fmt, ...);
+extern int sha1close(struct sha1file *);
+extern int sha1write(struct sha1file *, void *, unsigned int);
+extern int sha1write_compressed(struct sha1file *, void *, unsigned int);
+
+#endif