Feature request: External grep support

[Marcus Harnisch]

Hi Shigio

Until now I hadn't noticed that global has its own grep implementation.

It turns out that running ‘grep -l’ with a fixed string pattern on the list of files obtained with ‘global -P’ consumes about a quarter of the time of the corresponding ‘global -g’ command in a non-trivial code base (emacs-master, total size of files searched amounting to 79MB).

Modern grep replacements, such as ripgrep (https://github.com/BurntSushi/ripgrep) are even faster than that. Exact numbers will obviously differ based on the actual pattern used.

Perhaps grep() could be changed to off-loading to an external tool (possibly even configurable via gtags.conf)

Thanks,

Marcus