ignore: support backslash escaping

Use the new `Globset::backslash_escape` knob to conform to git behavior:
`\` will escape the following character. For example, the pattern `\*`
will match a file literally named `*`.

Also tweak a test in ripgrep that was relying on this incorrect
behavior.

Closes #526, Closes #811

ignore/src/gitignore.rs

@@ -477,6 +477,7 @@ impl GitignoreBuilder {
             GlobBuilder::new(&glob.actual)
                 .literal_separator(literal_separator)
                 .case_insensitive(self.case_insensitive)
+                .backslash_escape(true)
                 .build()
                 .map_err(|err| {
                     Error::Glob {
@@ -635,6 +636,10 @@ mod tests {
     ignored!(ig35, "./.", ".a/b", ".a/b");
     ignored!(ig36, "././", ".a/b", ".a/b");
     ignored!(ig37, "././.", ".a/b", ".a/b");
+    ignored!(ig38, ROOT, "\\[", "[");
+    ignored!(ig39, ROOT, "\\?", "?");
+    ignored!(ig40, ROOT, "\\*", "*");
+    ignored!(ig41, ROOT, "\\a", "a");
 
     not_ignored!(ignot1, ROOT, "amonths", "months");
     not_ignored!(ignot2, ROOT, "monthsa", "months");

tests/tests.rs

@@ -800,11 +800,7 @@ clean!(regression_25, "test", ".", |wd: WorkDir, mut cmd: Command| {
 
 // See: https://github.com/BurntSushi/ripgrep/issues/30
 clean!(regression_30, "test", ".", |wd: WorkDir, mut cmd: Command| {
-    if cfg!(windows) {
-        wd.create(".gitignore", "vendor/**\n!vendor\\manifest");
-    } else {
-        wd.create(".gitignore", "vendor/**\n!vendor/manifest");
-    }
+    wd.create(".gitignore", "vendor/**\n!vendor/manifest");
     wd.create_dir("vendor");
     wd.create("vendor/manifest", "test");