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

tests/multiline.rs

@@ -64,6 +64,11 @@ rgtest!(only_matching, |dir: Dir, mut cmd: TestCommand| {
 });
 
 // Tests that --vimgrep works in multiline mode.
+//
+// In particular, we test that only the first line of each match is printed,
+// even when a match spans multiple lines.
+//
+// See: https://github.com/BurntSushi/ripgrep/issues/1866
 rgtest!(vimgrep, |dir: Dir, mut cmd: TestCommand| {
     dir.create("sherlock", SHERLOCK);
     cmd.args(&[
@@ -77,7 +82,6 @@ rgtest!(vimgrep, |dir: Dir, mut cmd: TestCommand| {
     let expected = "\
 sherlock:1:16:For the Doctor Watsons of this world, as opposed to the Sherlock
 sherlock:1:57:For the Doctor Watsons of this world, as opposed to the Sherlock
-sherlock:2:1:Holmeses, success in the province of detective work must always
 sherlock:3:49:be, to a very large extent, the result of luck. Sherlock Holmes
 sherlock:5:12:but Doctor Watson has to have it taken out for him and dusted,
 ";