Improve function dir.c:trim_trailing_spaces()

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

Slightly more rare occurrences of 'text  \    ' with a backslash
in between spaces are handled correctly. Namely, the code in
8ba87adad6 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>