Make post-review warn if installed version of git-svn is too old

Review Request #343 — Created April 4, 2008 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
I installed git-svn the other day with an out-of-date MacPorts index, and wound up with version installed.  It turns out that 'git svn info', which is used by post-review to find repository information, wasn't added until git 1.5.4 (see ).  This adds a version check for git-svn which is run if git svn info fails, and which lets the user know that the git-svn version is out of date.
Tested against my old install and my new, hotter install.