coverage.c (bbg_file_stamp): New.

	* coverage.c (bbg_file_stamp): New.
	(read_counts_file): Merge incoming stamp with bbg_file_stamp.
	(build_info): Write bbg_file_stamp.
	(coverage_init): Initialize bbg_file_stamp.  Read counts file
	before writing graph header.
	(coverage_finish): Don't unlink the data file if we can generate a
	unique file stamp.
	* tree.h (crc32_unsigned): Declare.
	* tree.c (crc32_unsigned_bits): New, broken out of ...
	(crc32_byte): ... here.  Use it.
	(crc32_unsigned): New.

From-SVN: r189095