Bump version to 7.8.50.DATE-cvs.

Now that the GDB 7.8 branch has been created, we can
bump the version number.

gdb/ChangeLog:

	GDB 7.8 branch created (173373c6f6388171d1d62a217fae90a052395be2):
	* version.in: Bump version to 7.8.50.DATE-cvs.
Update NEWS post GDB 7.8 branch creation.

gdb/ChangeLog:

	* NEWS: Create a new section for the next release branch.
	Rename the section of the current branch, now that it has
	been cut.