Rename 'id' to 'sha' in data.frame from S4 class git_tree

* R/tree.r: Rename 'id' to 'sha' in data.frame coerced from S4 class
  git_tree.

* tests/tree.R: Add test case to check column names.

* man/coerce-git_tree-data.frame-method.Rd: Update documentation

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>