Reuse sha1 in packed_git in fast-import.

Rather than maintaing our own packfile level sha1 variable we
can make use of the one already available in struct packed_git.
Its meant for the SHA1 of the index but it can also hold the
SHA1 of the packfile itself between final checksumming of the
packfile and creation of the index.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -248,7 +248,6 @@ static struct packed_git *pack_data;
 static struct packed_git **all_packs;
 static int pack_fd;
 static unsigned long pack_size;
-static unsigned char pack_sha1[20];
 
 /* Table of objects we've written. */
 static unsigned int object_entry_alloc = 5000;
@@ -646,8 +645,8 @@ static void fixup_header_footer()
 	}
 	free(buf);
 
-	SHA1_Final(pack_sha1, &c);
-	write_or_die(pack_fd, pack_sha1, sizeof(pack_sha1));
+	SHA1_Final(pack_data->sha1, &c);
+	write_or_die(pack_fd, pack_data->sha1, sizeof(pack_data->sha1));
 }
 
 static int oecmp (const void *a_, const void *b_)
@@ -697,8 +696,8 @@ static void write_index(const char *idx_name)
 		sha1write(f, &offset, 4);
 		sha1write(f, (*c)->sha1, sizeof((*c)->sha1));
 	}
-	sha1write(f, pack_sha1, sizeof(pack_sha1));
-	sha1close(f, NULL, 1);
+	sha1write(f, pack_data->sha1, sizeof(pack_data->sha1));
+	sha1close(f, pack_data->sha1, 1);
 	free(idx);
 }