clone|--mirror: support --epoch=RANGE for partial clones
Partial (v2) clones should be useful addition for users wanting
to conserve storage while having fast access to recent messages.
Continuing work started in
876e74283ff3 (fetch: ignore
non-writable epoch dirs, 2021-09-17), this creates bare,
read-only epoch git repos. These git repos have the remotes
pre-configured, but does not fetch any objects.
The goal is to allow users to set the writable bit on a
previously-skipped epoch and start fetching it.
Shell completion support may not be necessary given how short
the epoch ranges are, here.
Cc: Luis Chamberlain <mcgrof@kernel.org>
Link: https://public-inbox.org/meta/20210917002204.GA13112@dcvr/T/#u