Add GIT_COLOR_BOLD_* and GIT_COLOR_BG_*

Add GIT_COLOR_BOLD_* and GIT_COLOR_BG_*

Add GIT_COLOR_BOLD_* macros to set both bold and the color in one
sequence.  This saves two characters of output ("ESC [ m", minus ";")
and makes the code more readable.

Add the remaining GIT_COLOR_BG_* macros to make the list complete.
The white and black colors are not included since they look bad on most
terminals.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>