gitk: Fix bug in generating patches

Commit 8f489363 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: NPaul Mackerras <paulus@samba.org>

gitk

@@ -5920,6 +5920,8 @@ proc mkpatchgo {} {
     set newid [$patchtop.tosha1 get]
     set fname [$patchtop.fname get]
     set cmd [diffcmd [list $oldid $newid] -p]
+    # trim off the initial "|"
+    set cmd [lrange $cmd 1 end]
     lappend cmd >$fname &
     if {[catch {eval exec $cmd} err]} {
 	error_popup "Error creating patch: $err"