printer: vimgrep now only prints one line

It turns out that the vimgrep format really only wants one line per
match, even when that match spans multiple lines.

We continue to support the previous behavior (print all lines in a
match) in the `grep-printer` crate. We add a new option to enable the
"only print the first line" behavior, and unconditionally enable it in
ripgrep. We can do that because the option has no effect in single-line
mode, since, well, in that case matches are guaranteed to span one line
anyway.

Fixes #1866

crates/core/args.rs

@@ -777,6 +777,7 @@ impl ArgMatches {
             .path(self.with_filename(paths))
             .only_matching(self.is_present("only-matching"))
             .per_match(self.is_present("vimgrep"))
+            .per_match_one_line(true)
             .replacement(self.replacement())
             .max_columns(self.max_columns()?)
             .max_columns_preview(self.max_columns_preview())