ripgrep: fix bug when CWD has directory named -

Specifically, when searching stdin, if the current directory has a
directory named `-`, then the `--with-filename` flag would automatically
be turned on. This is because `--with-filename` is automatically enabled
when ripgrep is given a single path that is a directory. When ripgrep is
given empty arguments, and if it is searching stdin, then its default
path list is just simple `["-"]`. The `is_dir` check passes, and
`--with-filename` gets enabled.

This commit fixes the problem by checking whether the path is `-` first.
If so, then we assume it isn't a directory. This is fine, since if it is
a directory and one asks to search it explicitly, then ripgrep will
interpret `-` as stdin anyway (which is arguably a bug on its own, but
probably not one worth fixing).

Fixes #1223, Closes #1292

src/args.rs

@@ -1594,10 +1594,11 @@ impl ArgMatches {
         if self.is_present("no-filename") {
             false
         } else {
+            let path_stdin = Path::new("-");
             self.is_present("with-filename")
             || self.is_present("vimgrep")
             || paths.len() > 1
-            || paths.get(0).map_or(false, |p| p.is_dir())
+            || paths.get(0).map_or(false, |p| p != path_stdin && p.is_dir())
         }
     }
 }