Emit information about reindexing git revision ranges when used
with xcpdb. Additionally, distinguish Xapian copy output from
v2 git epoch counting by increasing directory context info.
For now, v1 batches batches are emitted. v2 indexing is still
missing progress reporting for batches, as the data structures
for reindexing would benefit from a refactoring, first.
This does not currently affect the use of public-inbox-index,
but may in the future.