dir.c:trim_trailing_spaces(): fix for "\" sequence

Discard the unnecessary 'nr_spaces' variable, remove 'strlen()' and
improve the 'if' structure. Switch to pointers instead of integers
to control the loop.

Slightly more rare occurrences of 'text  \    ' with a backslash
in between spaces are handled correctly. Namely, the code in
7e2e4b37 (dir: ignore trailing spaces in exclude patterns, 2014-02-09)
does not reset 'last_space' when a backslash is encountered and the above
line stays intact as a result.

Add a test at the end of t/t0008-ignores.sh to exhibit this behavior

Signed-off-by: Pasha Bolokhov <pasha.bolokhov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>