`get_git_head_revision` does not work on submodule

[diggit]

Here variable ${GIT_EXECUTABLE} is used, but find_package(Git QUIET) is not called beforehand, thus it's empty resulting in empty ${out} and so on.

[rpavlik]

Can you just add find_package(Git QUIET) in your CMake scripts? For some reason I assumed that was already required.

[diggit]

That is probably also an option, but for example git_describe, git_describe_working_tree, git_local_changes,.. handle this by themselves. Thus it's a bit non systematic to auto find git in some function a not in others. IMO this should be unified - none or all.