GitPlugin: Improve performance

[Peter Suter]

Created as part of the move of GitPlugin. Tickets originally reported for ​th:GitPlugin: #746

Quoting ​hvr:

• try to create an additional libgit-thin-based Storage in PyGIT.py to evaluate whether the exec+fork+parse overhead is still signifikant
• most external git calls can be avoided now thanks to extensive caching of meta-data; one of the remaining speed killers is listing directories in the source browser (cat-file -s calls can be optimized, by using GIT 1.5.3+ -l option to ls-tree; but having to call rev-list for each folder element still remains an issue… maybe it'll be possible to get an enhancement to ls-tree merged upstream to provide that information as well…)

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See also: TracDev/Performance/Git

[Christian Boos]

Also, the 3 commits from last November in ​https://github.com/andersk/trac-git-plugin/commits/deforkify seem to be very interesting. Sorry, they're already in… looked in the wrong place.