The git_deploy module requires the git command to be executable by the
webserver.

An alternative to using git_deploy is to install modules and themes using the
"drush pm-download" command with options "--package-handler=git_drupalorg
--gitinfofile". This performs a git clone and checkout and then inserts the
desired version information into the .info file. The "drush make" command will
automatically write packaging information without additional options.