Rename S4 class hex slots to sha

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

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

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rename 'hex' to 'sha' in R methods

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

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rename 'hex' to 'sha' in git2r C code

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rename 'hex' to 'sha' in test cases

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