Add missing ChangeLog entries.
Bump version to 7.7.50.DATE-cvs.

Now that the GDB 7.7 branch has been created, we can
bump the version number.

gdb/ChangeLog:

	GDB 7.7 branch created (79301218fa0f074c5656db0ec8972a5ddcf91fb5):
	* version.in: Bump version to 7.7.50.DATE-cvs.
Update NEWS post GDB 7.7 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.