Fix bug with empty inc and exc
This fixes an exception that showed up when importing patches
+ hg init
+ hg add a
+ hg commit -m commit #0 -d 0 0
+ hg add b
+ hg commit -m commit #1 -d 0 0
+ hg update 0
+ hg add b
+ hg commit -m commit #2 -d 0 0
+ hg init
+ hg add a
+ hg commit -m commit #0 -d 0 0
+ hg add b
+ hg commit -m commit #1 -d 0 0
+ hg update 0
+ hg commit -A -m commit #2 -d 0 0
adding b
+ hg init
+ hg add a
+ hg commit -m commit #0 -d 0 0
+ hg add b
+ hg commit -m commit #1 -d 0 0
+ hg remove b
+ hg update 0
+ hg commit -A -m commit #2 -d 0 0
adding b