Support prefetch branches

[silvio]

Certain tasks can be automated using “git maintenance”. For example, it is possible to update the “commit-graph database” hourly or to automate “incremental repack” tasks daily. Another feature is to update all branches every hour.
Exactly with the last point I have noticed a problem, “tig” does not show me a name for “prefetched branches”, but displays these commits as “ghost branches/commits”, when the branch is removed on the server, but the maintenance run has not yet taken place.

gitk view:

tig view:

Manual page for git maintenance: https://man7.org/linux/man-pages/man1/git-maintenance.1.html

[koutcher]

Add set reference-format = [prefetch] to your .tigrc to see prefetch references.