docbook-manual-repo.py: improve logging

Display the last successful build's date (from the modified timestamp of
the '.build' flag file) when skipping build if there was no change in
the repository since that time.

To achieve this, the flag file's modified date is updated to current
time when we're not building the manuals.
docbook-manual-repo.py: only build when docbook source has changed

Restrict git log parsing to 'docbook' directory when retrieving last
commit timestamp to determine if we need to build the manuals. This
avoids unnecessarily running the build process.
docbook-manual-repo.py: revert to originally checked-out branch

Previously the script was leaving the repository checked-out to the last
processed ref.