Fix inconsistent placement of header-lines with border options

fzf displayed --header-lines inconsistently depending on the presence of borders:

  # --header and --header-lines co-located
  seq 10 | fzf --header-lines 3 --header "$(seq 101 103)" --header-first

  # --header and --header-lines separated
  seq 10 | fzf --header-lines 3 --header "$(seq 101 103)" --header-first --header-lines-border

This commit fixes the inconsistency with the following logic:

* If only one of --header or --header-lines is provided, --header-first
  applies to that single header.
* If both are present, --header-first affects only the regular --header,
  not --header-lines.

test/test_layout.rb

@@ -39,11 +39,11 @@ class TestLayout < TestInteractive
     tmux.send_keys "seq 1000 | #{FZF} --header foobar --header-lines 3 --header-first", :Enter
     block = <<~OUTPUT
       > 4
-        997/997
-      >
         3
         2
         1
+        997/997
+      >
         foobar
     OUTPUT
     tmux.until { assert_block(block, it) }
@@ -53,10 +53,10 @@ class TestLayout < TestInteractive
     tmux.send_keys "seq 1000 | #{FZF} --header foobar --header-lines 3 --header-first --reverse --inline-info", :Enter
     block = <<~OUTPUT
         foobar
+      >   < 997/997
         1
         2
         3
-      >   < 997/997
       > 4
     OUTPUT
     tmux.until { assert_block(block, it) }
@@ -148,10 +148,10 @@ class TestLayout < TestInteractive
       │
       │   4
       │ > 3
-      │   2/2
-      │ >
       │   2
       │   1
+      │   2/2
+      │ >
       │   foo
       ╰───────
     OUTPUT
@@ -609,11 +609,11 @@ class TestLayout < TestInteractive
       │   4
       │ > 3
       ╰──────────
+          2
+          1
           98/98 ─
         >
       ╭──────────
-      │   2
-      │   1
       │   hello
       ╰──────────
     BLOCK
@@ -666,12 +666,12 @@ class TestLayout < TestInteractive
       │   4
       │ > 3
       ╰──────────
+          98/98 ─
+        >
       ╔══════════
       ║   2
       ║   1
       ╚══════════
-          98/98 ─
-        >
     BLOCK
     tmux.until { assert_block(block1, it) }