trim_last_path_component(): avoid hard-coding the directory separator

trim_last_path_component(): avoid hard-coding the directory separator

Currently, this function hard-codes the directory separator as the
forward slash.

However, on Windows the backslash character is valid, too. And we want
to call this function in the upcoming support for symlinks on Windows
with the symlink targets (which naturally use the canonical directory
separator on Windows, which is _not_ the forward slash).

Prepare that function to be useful also in that context.

Signed-off-by: Karsten Blees <karsten.blees@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>