Ignore .sl directory the same way as .hg.

The .sl directory is used by Sapling (https://sapling-scm.com/) so
it should be handled like the other SCM directories.

Signed-off-by: Carmen Bianca BAKKER <carmenbianca@fsfe.org>

CHANGELOG.md

@@ -54,6 +54,8 @@ CLI command and its behaviour. There are no guarantees of stability for the
   currently exits early. To automatically create a .license file for
   unrecognised files, `--fallback-dot-license` has been added. (#823, #851,
   #853, #859; this took a while to get right.)
+- Ignore `.sl` directory as used by [Sapling SCM](https://sapling-scm.com/).
+  (#867)
 
 ### Changed
 

src/reuse/__init__.py

@@ -54,6 +54,7 @@
 _IGNORE_DIR_PATTERNS = [
     re.compile(r"^\.git$"),
     re.compile(r"^\.hg$"),
+    re.compile(r"^\.sl$"),  # Used by Sapling SCM
     re.compile(r"^LICENSES$"),
     re.compile(r"^\.reuse$"),
 ]