manifest.c: new extension code to lazily parse manifests This lets us iterate manifests in order, but do a _lot_ less work in the common case when we only care about a few manifest entries. Many thanks to Mike Edgar for reviewing this in advance of it going out to the list, which caught many things I missed. This version of the patch includes C89 fixes from Sean Farley and many correctness/efficiency cleanups from Martin von Zweigbergk. Thanks to both!

workingctx: use normal dirs() instead of dirstate.dirs() The workingctx class was using dirstate.dirs() as it's implementation. The sparse extension maintains a pruned down version of the dirstate, so this resulted in the workingctx reporting an incorrect listing of directories during merge calculations (it was detecting directory renames when it shouldn't have). The fix is to use the default implementation, which uses workingctx._manifest, which unions the manifest with the dirstate to produce the correct overall picture. This also produces more accurate output since it will no longer return directories that have been entirely deleted in the dirstate. Tests will be added to the sparse extension to detect regressions for this.

merge with stable

subrepo: only fetch unknown files from git when explicitly requested