Allow the changegroup generator to completely load the buffer.
This makes is possible to delete or truncate away revisions and still
use the changegroup later.
+ 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 c
+ hg commit -m commit #2 -d 0 0
+ hg update -m 1
+ 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 c
+ hg commit -m commit #2 -d 0 0
+ hg update -m 1
merging for b
merging 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 commit -m commit #2 -d 0 0
+ hg update 1
+ hg add c
+ hg commit -m commit #3 -d 0 0
This is file b1
+ hg update -m 2
merging for b
merging 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 commit -m commit #2 -d 0 0
+ hg update 1
+ hg add c
+ hg commit -m commit #3 -d 0 0
+ hg update -m 2
merging for b
merging b