generate_pack refreshed the data directories before probing its output
lock. The refresh waits for every run holding a tree, and a pipeline holds
them all: a second build sat silent for hours instead of being refused.
The output lock is now taken first, and a refresh that has to wait for a
tree names it.
refresh_data_dirs printed one line per entry and no total, a dry run named
the cached tag only, and cutting a quoted ETag at 12 characters kept one
quote. The run now ends on refreshed / up to date / failed counts, a dry
run shows the remote tag beside the cached one, and tags lose their quotes
before they are cut.
Abdessamad Derraz