Document config option format.usefncache and improve format.usestore docs.

add format.usefncache config option (default is true) This enables for example "hg clone --config format.usefncache=0" which creates a non-fncache repository, which then can be read by Mercurial versions before 7946503ec76e.

fix pull racing with push/commit (issue1320) changegroup() has a problem when nodes which does not descend from a node in <bases> are added to remote after the discovery phase. If that happens, changegroup() won't send the correct set of nodes, ie. some nodes will be missing. To correct it we have to find the set of nodes that both remote and self have (called <common>), and send all the nodes not in <common>. This fix has some overhead, in the worst case it will re-send a whole branch. A proper fix to avoid this overhead might be to change the protocol so that the <common> nodes are sent (instead of the <bases> of the missing nodes).

convert: correctly detect missing revlog for root revisions

convert: ignore hg source errors with hg.ignoreerrors (issue 1357) This flag makes missing revlog errors to be ignored which allow broken repositories to be fixed by converting them from and to Mercurial.

grep: avoid infinite loop when trailing newline is missing