gitk: Fix bug in generating patches

gitk: Fix bug in generating patches

Commit 8f4893639129acfc866c71583317090aa2a46eab changed mkpatchgo
to use diffcmd rather than constructing the diff command itself.
Unfortunately diffcmd returns the command with a "|" as the first
element (ready for use with open), but exec won't accept the "|".
Thus we need to remove the "|".

Signed-off-by: Paul Mackerras <paulus@samba.org>