GTags single file update feature proposal

[Pavel Nedev]

Hi,

I was experimenting recently with gtags --single-update option and I noticed 
that if the file given does not exist gtags returns error message and does 
nothing more.

Here is the specific use case:

- I have project database generated with 'gtags -c'
- I delete file from the project (let's say it is test.cpp) 
- I want to update database only for this file as only it is changed 
(completely removed) so I do:

gtags -c --single-update test.cpp

At this point gtags returns error message saying that the file does not exist.
I was expecting that gtags will update the database removing all references for 
test.cpp (path from GPATH and all symbols found inside test.cpp).
I don't know if this is feasible but it would be very nice feature (and is very 
intuitive also IMO).

Thank you for the support!

Best Regards,
Pavel