Stream results progressively [FCOM-8]

This change brings back streaming of results, which I think was broken
in #639 .

Unsurprisingly, given the complexity/overhead added to the
implementation, the fix herein does hurt performance somewhat, but not
terribly. I tested with the following queries in `david_runger` and got:

1. **with this change:** `time /home/david/code/fcom/exe/fcom
   javascript` => 15.1s, 14.6s

2. **with latest release:** `time fcom javascript` => 12.9s, 12.7s

I think that this is well worthwhile, in order to be able to start
seeing streaming results as soon as they begin to become available
(which often allows for Ctrl-C to interrupt the command before it
finishes, anyway, resulting in a net time savings, even if it is slower
if/when run all the way to completion).

Private reference link:
https://chat.deepseek.com/a/chat/s/6eb15fc5-626f-48b9-b329-a0fd6933d96a

Author: davidrunger

CHANGELOG.md

@@ -1,5 +1,5 @@
 ## Unreleased
-[no unreleased changes yet]
+- Stream results progressively.
 
 ## v0.14.0 (2024-12-10)
 - Remove upper bounds on versions for all dependencies.

lib/fcom/querier.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'pty'
+
 require_relative 'options_helpers.rb'
 
 # This class executes a system command to retrieve the git history, which is passed through `rg`
@@ -63,18 +65,34 @@ def query
 
     commands.each do |command|
       Fcom.logger.debug("Executing command: #{command}")
-      output = `#{command}`
 
-      if output.empty?
-        previous_command_generated_output = false
-      else
-        if previous_command_generated_output
-          puts("\n\n") # print blank lines for spacing
-        end
+      # Add spacing if needed
+      if previous_command_generated_output
+        print "\n\n"
+      end
+
+      PTY.spawn(command) do |stdout, _stdin, _pid|
+        any_bytes_seen_for_command = false
+
+        # Read first byte to detect any output
+        first_byte = stdout.read(1)
 
-        previous_command_generated_output = true
+        any_bytes_seen_for_command = true
 
-        puts(output)
+        if first_byte
+          previous_command_generated_output = true
+
+          print(first_byte)
+
+          # Now read the rest line by line
+          stdout.each_line { puts(it) }
+        else
+          previous_command_generated_output = false
+        end
+      rescue Errno::EIO
+        if !any_bytes_seen_for_command
+          previous_command_generated_output = false
+        end
       end
     end
   end