cli: fix stdin detection for Powershell on Unix

It seems that PowerShell uses sockets instead of FIFOs to redirect the
output between commands. So add `is_socket` to our `is_readable_stdin`
check.

This seems unlikely to cause problems and it probably more generally
correct than what we had before. In theory, it could cause problems if
it produces false positives, in which case, ripgrep will try to read
stdin when it should search the current working directory. (And this
usually winds up manifesting as ripgrep blocking forever.) But, if the
stdin handle reports itself as a socket, then it seems like we should
read it.

Fixes #1741, Closes #1742

CHANGELOG.md

@@ -19,6 +19,8 @@ Bug fixes:
 
 * [BUG #1277](https://github.com/BurntSushi/ripgrep/issues/1277):
   Document cygwin path translation behavior in the FAQ.
+* [BUG #1741](https://github.com/BurntSushi/ripgrep/issues/1741):
+  Fix stdin detection when using PowerShell in UNIX environments.
 
 
 12.1.1 (2020-05-29)

crates/cli/src/lib.rs

@@ -210,7 +210,7 @@ pub fn is_readable_stdin() -> bool {
             Err(_) => return false,
             Ok(md) => md.file_type(),
         };
-        ft.is_file() || ft.is_fifo()
+        ft.is_file() || ft.is_fifo() || ft.is_socket()
     }
 
     #[cfg(windows)]