Fix line-wise autosuggestion false positive when line doesn't start command

To reduce the likelihood of false positive line-wise history
autosuggestions, we only them when the cursor's line starts a new
process ("parse_util_process_extent").

There are still false positives. Given

	$ true &&
          somecommand
	$ echo "
	someothercommand
	"

typing "some" suggests "someothercommand" from history even though
that was not actually used as command.

Fix this by using similar rules for suggestion candidates.

Might help #12290

tests/pexpects/autosuggest.py

@@ -80,3 +80,11 @@ run("rm configure")
 send("./con")
 use_suggestion()
 expect_prompt(">./con\r\n")
+
+send(control("c"))
+run("touch somecommand")
+run("chmod +x somecommand")
+run('echo "multiline-token\n./somecommand arg1 arg2"')
+send("./some")
+use_suggestion()
+expect_prompt(">./somecommand \r\n")