WIP: Add support for git-style .gitignore patterns to zoekt-index.

[kisabaka]

Adds support for .gitignore type globs to the zoekt-index command. This is useful when indexing a local directory or a git repository that doesn't follow one of the remote patterns defined in setTemplates in gitindex/index.go.

Usage:

go run cmd/zoekt-index/main.go -ignore_files .gitignore -index /tmp/zoekt-index/ .