manifest: document return type of readfast()
I keep having to ponder out what readfast() means, and it always
surprises me. Document the return type in the docstring so that future
readers won't have to puzzle this out again.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a