Fix incompatibility of adaptive height and 'start:reload'

This command would cause a deadlock and make fzf crash:

  fzf --bind 'start:reload:ls' --height ~100%

Because,

1. 'start' event is handled by Terminal
2. When 'reload' is bound to 'start', fzf avoids starting the initial reader
3. Terminal waits for the initial input to find the right height when
   adaptive height is used
4. Because the initial reader is not started, Terminal never gets the
   initial list
5. No chance to trigger 'start:reload', hence deadlock

This commit fixes the above problem by extracting the reload command
bound to 'start' event and starting the initial reader with that command
instead of letting Terminal start it.

This commit also makes the environment variables available to
$FZF_DEFAULT_COMMAND.

  FZF_DEFAULT_COMMAND='echo $FZF_QUERY' fzf --query foo

Fix #3944

CHANGELOG.md

@@ -1,6 +1,14 @@
 CHANGELOG
 =========
 
+0.54.3
+------
+- Fixed incompatibility of adaptive height and 'start:reload'
+- Environment variables are now available to `$FZF_DEFAULT_COMMAND`
+  ```sh
+  FZF_DEFAULT_COMMAND='echo $FZF_QUERY' fzf --query foo
+  ```
+
 0.54.2
 ------
 - Fixed incorrect syntax highlighting of truncated multi-line entries