bug#14849: (system vm coverage) `line-execution-counts' misleading output

[Taylan Ulrich B.]

The following shell/guile transcript showcases a minimal test-case for
when a tail-call fools the code-coverage module into thinking that the
line with the tail-call was not executed.

The result in the Guile transcript, $1, is an alist from line numbers
(starting from zero) to execution counts.

Converting the relevant coverage-data object into LCOV data via
`coverage-data->lcov', as advertised in (info "(guile) Code
Coverage"), then to HTML via LCOV's genhtml(1), will result in the
relevant line being colored red and contribute negatively to the total
line-coverage percentage.

=== BEGIN TRANSCRIPT ===
$ cat /tmp/test.scm  
(define (tail-call-test)
  (display "foo\n")
  (tail-call-target))

(define (tail-call-target)
  (display "bar\n"))
$ guile              
GNU Guile 2.0.9.20-10454
Copyright (C) 1995-2013 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (begin
...   (load "/tmp/test.scm")
...   (call-with-values
...       (lambda ()
...         ((@ (system vm coverage) with-code-coverage)
...          ((@ (system vm vm) the-vm))
...          tail-call-test))
...     (lambda (data . values)
...       ((@ (system vm coverage) line-execution-counts) data 
"/tmp/test.scm"))))
;;; <stdin>:0:4095: warning: possibly unbound variable `tail-call-test'
foo
bar
$1 = ((5 . 1) (4 . 1) (2 . 0) (1 . 1) (0 . 1))
scheme@(guile-user)> 
=== END TRANSCRIPT ===