Use the default git colouring scheme rather than specific scripts

This patch adds the mechanism to check if the output is tty for the
diff and show commands and passes the --color option to git if the
color.diff config option is set auto or true. The patch also changes the
default pager to 'less -FRSX' from the diffcol.sh script.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>