wt-status: don't find scissors line beyond buf len

wt-status: don't find scissors line beyond buf len

If

  (a) There is a "---" divider in a commit message,

  (b) At some point beyond that divider, there is a cut-line (that is,
      "# ------------------------ >8 ------------------------") in the
      commit message,

  (c) the user does not explicitly set the "no-divider" option,

then "git interpret-trailers" will hang indefinitively.

This is because when (a) is true, find_end_of_log_message() will invoke
ignored_log_message_bytes() with a len that is intended to make it
ignore the part of the commit message beyond the divider. However,
ignored_log_message_bytes() calls wt_status_locate_end(), and that
function ignores the length restriction when it tries to locate the cut
line. If it manages to find one, the returned cutoff value is greater
than len. At this point, ignored_log_message_bytes() goes into an
infinite loop, because it won't advance the string parsing beyond len,
but the exit condition expects to reach cutoff.

Make wt_status_locate_end() honor the length parameter passed in, to
fix this issue.

In general, if wt_status_locate_end() is given a piece of the memory
that lacks NUL at all, strstr() may continue across page boundaries
and run into an unmapped page.  For our current callers, this is not
a problem, as all of them except one uses a memory owned by a strbuf
(which guarantees an implicit NUL-termination after its payload),
and the one exception in trailer.c:find_end_of_log_message() uses
strlen() to compute the length before calling this function.

Signed-off-by: Florian Schmidt <flosch@nutanix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@nutanix.com>
[jc: tweaked the commit log message and the implementation a bit]
Signed-off-by: Junio C Hamano <gitster@pobox.com>