Calling revlog.addgroup with an empty changegroup now raises RevlogError. The empty changegroup can be caused by remote servers dying soon after findincoming, and further code in pull assumes (correctly) that there are new changesets.

adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
! a.c
! dir/a.o
! dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c