introduce fncache repository layout
* adds a new entry 'fncache' to '.hg/requires' for new repos
* writes new file '.hg/store/fncache'
* hash-encodes filenames with long paths (issue839)
* encodes Windows reserved filenames (issue793)
% prepare repo
adding FOO.txt
adding QUICK.txt
adding bar.txt
% verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
3 files, 1 changesets, 3 total revisions
% introduce some bugs in repo
% verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
data/FOO.txt.i@0: missing revlog!
0: empty or missing FOO.txt
FOO.txt@0: f62022d3d590 in manifests not found
data/QUICK.txt.i@0: missing revlog!
0: empty or missing QUICK.txt
QUICK.txt@0: 88b857db8eba in manifests not found
data/bar.txt.i@0: missing revlog!
0: empty or missing bar.txt
bar.txt@0: 256559129457 in manifests not found
3 files, 1 changesets, 0 total revisions
9 integrity errors encountered!
(first damaged changeset appears to be 0)