config_file_example: shell command should be cmd.exe on Windows

and add shell command debug in logging

docs/config_file_example.yaml

@@ -28,5 +28,8 @@ finder:
 # tags: git,!checkout # equivalent to the --tag-rules option
 
 shell:
-  command: bash # shell used for shell out. possible values: bash, zsh, dash, ...
+  # Shell used for shell out. Possible values: bash, zsh, dash, ...
+  # For Windows, use `cmd.exe` instead.
+  command: bash
+  
   # finder_command: bash # similar, but for fzf's internals

src/common/shell.rs

@@ -42,5 +42,6 @@ pub fn out() -> Command {
     cmd.args(words);
     let dash_c = if words_str.contains("cmd.exe") { "/c" } else { "-c" };
     cmd.arg(dash_c);
+    debug!("shell cmd = `{cmd:#?}`");
     cmd
 }