[ANNOUNCE] public-inbox.org/git/ search updates for diffs

Hey all,

https://public-inbox.org/git/_/text/help has a few new prefixes
which might help improve searching:

        dfn:     match filename from diff
        dfa:     match diff removed (-) lines
        dfb:     match diff added (+) lines
        dfhh:    match diff hunk header context (usually a function name)
        dfctx:   match diff context lines
        dfpre:   match pre-image git blob ID
        dfpost:  match post-image git blob ID
        dfblob:  match either pre or post-image git blob ID

The blob ID searches should work down to 7 characters right now.

I updated the xapian indices in place ( "public-inbox-index --reindex" ),
so it should've happened without a service interruption.

Probably a couple more changes on the way this month depending
on my schedule and workload on other stuff.