scm: git: add test of revisions() in merged revisions in unit adapter test (#7146).

scm: git: add test of revisions() in merged revisions in unit adapter test (#7146).

Git branch is a reference of specific revision.
Git branch is equivalent with Mercurial *heads*.
http://mercurial.selenic.com/wiki/GitConcepts?action=recall&rev=53#Branch_model

32ae898b7 in git test repository is a merged revision with two heads.
713f49446 and 4a07fe31b *were* in different branches before merged.
713f49446 and 4a07fe31b are descendants of 61b685fbe5.
4a07fe31b is a child of 61b685fbe5.
To get revisions from 713f4944648826f5 to master,
"git log" needs to return 4a07fe31b.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@5760 e93f8b46-1217-0410-a6f0-8f06a7374b81