gitk: Fixed broken exception handling in diff

If the tree diff command failed to start for some
random reason, treepending remained set, and thus
no more diffs were shown after that.

Signed-off-by: Alexander Gavrilov <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>

Author: angavrilov

gitk

@@ -6457,9 +6457,10 @@ proc diffcmd {ids flags} {
 proc gettreediffs {ids} {
     global treediff treepending
 
+    if {[catch {set gdtf [open [diffcmd $ids {--no-commit-id}] r]}]} return
+
     set treepending $ids
     set treediff {}
-    if {[catch {set gdtf [open [diffcmd $ids {--no-commit-id}] r]}]} return
     fconfigure $gdtf -blocking 0
     filerun $gdtf [list gettreediffline $gdtf $ids]
 }