insert: strip trailing / in folder path

insert: strip trailing / in folder path

This patch fixes the "Insert message into folder with trailing /"
test. The problem was insufficient path canonization.

From database's point of view, "Sent" and "Sent/" are different
folders. If user runs (note the last '/'):

    notmuch insert --folder=maildir/Sent/ < test.msg

notmuch will create an extra XDIRECTORY record for the folder
'Sent/'. This means that database will have _TWO_ records for _ONE_
physical folder: 'Sent' and 'Sent/'. However, the 'notmuch new'
command will update only records related to the first one (the correct
one).

Now, if user moved the email file (e.g. from 'Sent/new' to
'Sent/cur'), 'notmuch new' will add a record about the new file, but
will not delete the old record.

Signed-off-by: Yuri Volchkov <yuri.volchkov@gmail.com>