httprepo: decompress stream incrementally to reduce memory usage

verify: initialize rp variable in case we hit out of memory

merge: drop reference to file contents after write This reduces memory usage on large consecutive gets.

revlog: drop cache after use to save memory footprint If we reconstruct back to back large versions, we need to drop the cache first to avoid doubling memory usage.

verify: reduce memory footprint when unpacking files By never holding a reference to the unpacked string, we avoid holding two consecutive large files in memory.

verify: report exceptions that don't have a str() method