gitk: work around ridiculous command line restriction on Windows
On Windows, there are dramatic problems when a command line grows
beyond PATH_MAX, which is restricted to 255 characters.
Work around this by just cutting off the command line at that length
(actually, at a space boundary) in the hope that only negative
refs are chucked: gitk will then do unnecessary work, but that is
still better than flashing the gitk window and exiting with exit
status 5 (which no Windows user is able to make sense of).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>