The following patch adds a new user-visible variable to GDB, 
'max-completion-results'.

It is used by the completion functions in symtab.c to limit the maximum 
number of results returned by the completion function, and defaults to 
'unlimited'.  I find it particuarly useful when using GDB under emacs, 
to prevent situations where hitting <tab> at the wrong time can lead to 
hundreds of thousands of results having to be sorted by 
complete_command.