Respect gitignores for both .git and .sl directories

Summary: rg only respects gitignores for directories containing or with a
parent containing a `.git` directory.
[Sapling](https://sapling-scm.com/docs/introduction/) is a VCS tool that works
with git repositories (and is what I'm using to make this commit), but uses
`.sl` instead of `.git`.  This commit respect gitignores for both `.git` and
`.sl` directories

Test Plan: `tests/misc.rs`

crates/ignore/src/dir.rs

@@ -204,7 +204,7 @@ impl Ignore {
             igtmp.absolute_base = Some(absolute_base.clone());
             igtmp.has_git =
                 if self.0.opts.require_git && self.0.opts.git_ignore {
-                    parent.join(".git").exists()
+                    parent.join(".git").exists() || parent.join(".sl").exists()
                 } else {
                     false
                 };
@@ -796,7 +796,7 @@ fn resolve_git_commondir(
     git_type: Option<FileType>,
 ) -> Result<PathBuf, Option<Error>> {
     let git_dir_path = || dir.join(".git");
-    let git_dir = git_dir_path();
+    let git_dir = git_dir_path()
     if !git_type.map_or(false, |ft| ft.is_file()) {
         return Ok(git_dir);
     }

tests/misc.rs

@@ -578,6 +578,29 @@ rgtest!(ignore_git, |dir: Dir, mut cmd: TestCommand| {
     cmd.assert_err();
 });
 
+
+rgtest!(uses_gitignore, |dir: Dir, mut cmd: TestCommand| {
+    dir.create("file1", "hi");
+    dir.create("file2", "hi");
+    dir.create_dir(".git");
+    dir.create(".gitignore", "file1\n");
+    cmd.arg("hi");
+
+    //cmd.assert_err();
+    eqnice!("file2:hi\n", cmd.stdout());
+});
+
+rgtest!(uses_gitignore_for_sapling, |dir: Dir, mut cmd: TestCommand| {
+    dir.create("file1", "hi");
+    dir.create("file2", "hi");
+    dir.create_dir(".sl");
+    dir.create(".gitignore", "file1\n");
+    cmd.arg("hi");
+
+    //cmd.assert_err();
+    eqnice!("file1:hi\n", cmd.stdout());
+});
+
 rgtest!(ignore_generic, |dir: Dir, mut cmd: TestCommand| {
     dir.create("sherlock", SHERLOCK);
     dir.create(".ignore", "sherlock\n");