add ExecutionContext's public variable when run git.exe (#952)
add ExecutionContext's public variable when run git.exe (#952)

since wrapper task may set variable before get source been called.
kasubram/vsts-agent,lkillgore/vsts-agent,Microsoft/vsts-agent,lkillgore/vsts-agent,kasubram/vsts-agent,kasubram/vsts-agent,Microsoft/vsts-agent,Microsoft/vsts-agent,lkillgore/vsts-agent,lkillgore/vsts-agent,Microsoft/vsts-agent,kasubram/vsts-agent