Replace custom GIT_IS_REPO macro with `git rev-parse --is-inside-work-tree`
Replace custom GIT_IS_REPO macro with `git rev-parse --is-inside-work-tree`

MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK,MITK/MITK