gitk: Remove closed file descriptors from $blobdifffd

gitk: Remove closed file descriptors from $blobdifffd

One shouldn't have descriptors of already closed files around.

The first idea to deal with this (previously) ever growing array
was to remove it entirely, but it's needed to detect start of a
new diff with ths old diff not yet done. This happens when a user
clicks on the same commit in the commit list repeatedly without
delay.

Signed-off-by: Markus Hitter <redacted>
Signed-off-by: Paul Mackerras <redacted>