scm: git: performance improvements in fetching revisions (#8857, #9472)

scm: git: performance improvements in fetching revisions (#8857, #9472)

Parse a revision for a given branch,
just if we haven't parsed it for any branches before.
Moved the db check to for existing revisions into a grouped search.
Search for many revisions at once: this reduces db load.
Revisions are grouped into sets of 100.
This is to improve memory consumption.
There will be just one query instead of each 100.
The above two methods significantly increase parsing speed.
Test case was a git repo with 6000+ commits on a master branch,
and several other branches originating for master.
Speed improved from 1.4h to 18min.

Contributed by Gergely Fábián.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@9144 e93f8b46-1217-0410-a6f0-8f06a7374b81