fish_clipboard_paste: Only trim leading whitespace on first line

We remove leading spaces so a paste isn't histignored, but we did so
on all lines, which removed indentation.

[ci skip]

share/functions/fish_clipboard_paste.fish

@@ -35,7 +35,7 @@ function fish_clipboard_paste
     if not string length -q -- (commandline -c)
         # If we're at the beginning of the first line, trim whitespace from the start,
         # so we don't trigger ignoring history.
-        set data (string trim -l -- $data)
+        set data[1] (string trim -l -- $data[1])
     end
     if test -n "$data"
         commandline -i -- $data