[boost] [git] Bumping git version requirement to 1.7.10

https://svn.boost.org/trac/boost/wiki/Git/GitHome documents Boost's version requirement for Git, currently at 1.7.3. The regression scripts are using the clone --single-branch option, which became available at git 1.7.10, so we are planning to bump the version requirement. Done anyone have a problem with this? --Beman