Do not perform cross-directory renames when creating packs

A comment on top of create_tmpfile() describes caveats ('can have
problems on various systems (FAT, NFS, Coda)') that should apply
in this situation as well.  This in the end did not end up solving
any of my personal problems, but it might be a useful cleanup patch
nevertheless.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-pack-objects.c

@@ -465,7 +465,7 @@ static void write_pack_file(void)
 			char tmpname[PATH_MAX];
 			int fd;
 			snprintf(tmpname, sizeof(tmpname),
-				 "%s/tmp_pack_XXXXXX", get_object_directory());
+				 "%s/pack/tmp_pack_XXXXXX", get_object_directory());
 			fd = xmkstemp(tmpname);
 			pack_tmp_name = xstrdup(tmpname);
 			f = sha1fd(fd, pack_tmp_name);

fast-import.c

@@ -816,7 +816,7 @@ static void start_packfile(void)
 	int pack_fd;
 
 	snprintf(tmpfile, sizeof(tmpfile),
-		"%s/tmp_pack_XXXXXX", get_object_directory());
+		"%s/pack/tmp_pack_XXXXXX", get_object_directory());
 	pack_fd = xmkstemp(tmpfile);
 	p = xcalloc(1, sizeof(*p) + strlen(tmpfile) + 2);
 	strcpy(p->pack_name, tmpfile);
@@ -878,7 +878,7 @@ static char *create_index(void)
 	}
 
 	snprintf(tmpfile, sizeof(tmpfile),
-		"%s/tmp_idx_XXXXXX", get_object_directory());
+		"%s/pack/tmp_idx_XXXXXX", get_object_directory());
 	idx_fd = xmkstemp(tmpfile);
 	f = sha1fd(idx_fd, tmpfile);
 	sha1write(f, array, 256 * sizeof(int));

index-pack.c

@@ -172,7 +172,7 @@ static char *open_pack_file(char *pack_name)
 		if (!pack_name) {
 			static char tmpfile[PATH_MAX];
 			snprintf(tmpfile, sizeof(tmpfile),
-				 "%s/tmp_pack_XXXXXX", get_object_directory());
+				 "%s/pack/tmp_pack_XXXXXX", get_object_directory());
 			output_fd = xmkstemp(tmpfile);
 			pack_name = xstrdup(tmpfile);
 		} else

pack-write.c

@@ -45,7 +45,7 @@ char *write_idx_file(char *index_name, struct pack_idx_entry **objects,
 	if (!index_name) {
 		static char tmpfile[PATH_MAX];
 		snprintf(tmpfile, sizeof(tmpfile),
-			 "%s/tmp_idx_XXXXXX", get_object_directory());
+			 "%s/pack/tmp_idx_XXXXXX", get_object_directory());
 		fd = xmkstemp(tmpfile);
 		index_name = xstrdup(tmpfile);
 	} else {