scan-tree: fix regression in section-from-path=-1

Commit fb3655d (use struct strbuf instead of static buffers -
2013-04-06) introduced a regression in the "section-from-path" handling
when the configured value is negative.  By changing the "rel" variable
so that it includes a trailing slash, counting slashes from the end of
the string no longer gives the same answer as it did before.

Fix this by ensuring that "rel" does not have a trailing slash.

Reported-by: Julius Plenz <plenz@cis.fu-berlin.de>
Signed-off-by: John Keeping <john@keeping.me.uk>