convert: move filemapper class to a separate file

convert: disable current --filemap support The --filemap support in hg convert doesn't handle merges correctly. (And after 33015dac5df5 I managed to break it even for simple cases where we don't want the first revision.)

convert: allow the converter_source to say "skip this revision" If getchanges returns a string, it's assumed to be the id of an already converted revision. We map the current revision to the same revision this converted revision was mapped to. To allow skipping a root revision, getchanges can return the special string 'hg-convert-skipped-revision' (a.k.a. common.SKIPREV), which hopefully won't clash with any real id. The converter_source is responsible for rewriting the parents of the commit objects to make sure the revision graph makes sense.

convert: pass the order of the revmapfile to the converter_source The entries in this file are supposed to be topologically sorted and this may be useful for a converter_source.

Merge with crew

merge: add debug diagnostics for findcopies

changegroup: avoid large copies - handle chunk headers separately rather than prepending them to (potentially large) chunks - break large chunks into 1M pieces for compression - don't prepend file metadata onto (potentially large) file data

revlog: generate trivial deltas against null revision To avoid extra memory usage and performance issues with large files, generate a trivial delta header for deltas against the null revision rather than calling the usual delta generator. We append the delta header to meta rather than prepending it to data to avoid a large allocate and copy.

Merge with crew

Merge with crew