[feature] Support printing context around matches

[joshtriplett]

⭐ Suggestion

grep and git grep support printing context lines around matches. -C 3 prints three lines of context before and after each match; -A 3 prints three lines after, and -B 3 prints three lines before.

I'd love to have the same functionality in ast-grep's sg run: sg -C 3 -p xyz.

As with grep and git grep, context lines should be printed slightly differently than matching lines. When printing context lines, grep and git grep print filename: before each matching line but filename- before each non-matching context line, and print a -- between matches. If a line in the context also matches the pattern, they print that line as filename: as well, and extend the context. For instance, for git grep -C3 matching:

...
--
file- context line
file- context line
file- context line
file: matching line
file- context line
file: another matching line
file- context line
file- context line
file- context line
--
...

💻 Use Cases

Having context makes it easier to skim through a series of matches. I use these options quite frequently with grep and git grep. Not having them in sg means I may need to check each match one by one, such as in an editor, rather than skimming through a series of them and only looking at ones with relevant context.

[HerringtonDarkholme]

I will add the feature in future releases. At the moment I will unblock the feature by these two issues. #512 #513

Stay tuned! Hopefully it can be released in ast-grep 0.10.0 or so.