vcs.rb: fix srcdir in VCS::GIT
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56668 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vcs.rb: fix srcdir in VCS::GIT
for IO.popen if srcdir is a local path. unless -srcdir command
line option is given, srcdir is the default URL.
[ruby-core:78036] [Bug #12908]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56668 b2dd03c8-39d4-4d8f-98ff-823fe69b080e