ignore: tweak regex crate features

This removes most of the Unicode features as they aren't currently
used. We can always add them back later if necessary.

We can avoid the unicode-perl feature by changing `\s` to `[[:space:]]`,
which uses the ASCII-only definition of `\s`. Since we don't expect
non-ASCII whitespace in git config files, this seems okay.

Closes #2502

crates/ignore/Cargo.toml

@@ -23,7 +23,7 @@ globset = { version = "0.4.10", path = "../globset" }
 lazy_static = "1.1"
 log = "0.4.5"
 memchr = "2.5"
-regex = "1.8.3"
+regex = { version = "1.9.0", default-features = false, features = ["perf", "std", "unicode-gencat"] }
 same-file = "1.0.4"
 thread_local = "1"
 walkdir = "2.2.7"

crates/ignore/src/gitignore.rs

@@ -596,8 +596,13 @@ fn parse_excludes_file(data: &[u8]) -> Option<PathBuf> {
     // probably works in more circumstances. I guess we would ideally have
     // a full INI parser. Yuck.
     lazy_static::lazy_static! {
-        static ref RE: Regex =
-            Regex::new(r"(?im)^\s*excludesfile\s*=\s*(.+)\s*$").unwrap();
+        static ref RE: Regex = Regex::new(
+            r"(?xim-u)
+            ^[[:space:]]*excludesfile[[:space:]]*
+            =
+            [[:space:]]*(.+)[[:space:]]*$
+            "
+        ).unwrap();
     };
     let caps = match RE.captures(data) {
         None => return None,