read-cache: save index SHA-1 after reading

Also update SHA-1 after writing. If we do not do that, the second
read_index() will see "initialized" variable already set and not read
.git/index again, which is fine, except istate->sha1 now has a stale
value.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/read-cache.c b/read-cache.c
index d1ad227..b4653c0 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1481,6 +1481,7 @@
 	if (verify_hdr(hdr, mmap_size) < 0)
 		goto unmap;
 
+	hashcpy(istate->sha1, (const unsigned char *)hdr + mmap_size - 20);
 	istate->version = ntohl(hdr->hdr_version);
 	istate->cache_nr = ntohl(hdr->hdr_entries);
 	istate->cache_alloc = alloc_nr(istate->cache_nr);
@@ -1616,7 +1617,7 @@
 		(ce_write(context, fd, &sz, 4) < 0)) ? -1 : 0;
 }
 
-static int ce_flush(git_SHA_CTX *context, int fd)
+static int ce_flush(git_SHA_CTX *context, int fd, unsigned char *sha1)
 {
 	unsigned int left = write_buffer_len;
 
@@ -1634,6 +1635,7 @@
 
 	/* Append the SHA1 signature at the end */
 	git_SHA1_Final(write_buffer + left, context);
+	hashcpy(sha1, write_buffer + left);
 	left += 20;
 	return (write_in_full(fd, write_buffer, left) != left) ? -1 : 0;
 }
@@ -1872,7 +1874,7 @@
 			return -1;
 	}
 
-	if (ce_flush(&c, newfd) || fstat(newfd, &st))
+	if (ce_flush(&c, newfd, istate->sha1) || fstat(newfd, &st))
 		return -1;
 	istate->timestamp.sec = (unsigned int)st.st_mtime;
 	istate->timestamp.nsec = ST_MTIME_NSEC(st);