[feature] Add relative path support to Git SCM tool

[nmasseyKM]

When trying to setup Conan packaging with the target source in a git submodule relative to the recipe, I was receiving
empty commit ids. Upon further investigation, it seems that the current assumption is that the folder path to Git()
will be absolute. A minor adjustment allows relative paths to be used as well: since the git command always chdirs to the
target git folder, we can perform the rev-parse relative to the current directory and receive the expected behavior.

Note that the current default value of "." is a relative path, which suggests that other relative paths should work.

• I've read the CONTRIBUTING guide.

[memsharded]

Closed by #11407.

Thanks for contributing it! Will be released in next 1.50 (and be part of Conan 2.0 beta, as we merge develop->develop2 continously)