This patch adds a --incremental-base option, so that the base file for
the incremental link can be different from the output file. For
simplicity, I simply copy the base file contents to a new output file,
then carry on with a normal incremental link.

-cary


	* gold.cc (queue_initial_tasks): Pass incremental base filename
	to Output_file::open_for_modification.
	* incremental-dump.cc (main): Adjust call to
	Output_file::open_for_modification.
	* incremental.cc (Incremental_inputs::report_command_line):
	Ignore --incremental-base option when comparing command lines.
	* options.h (class General_options): Add --incremental-base.
	* output.cc (Output_file::Output_file):
	(Output_file::open_for_modification): Add base_name parameter;
	copy base file to new file.
	* output.h (Output_file::open_for_modification): Add base_name
	parameter.
	* testsuite/Makefile.am (incremental_test_4): Test
	--incremental-base.
	* testsuite/Makefile.in: Regenerate.