[RFC PATCH] color: respect the $NO_COLOR convention

When the NO_COLOR environment variable is set to any value, default to
disabling color, i.e. resolve 'auto' to false.

NO_COLOR (http://no-color.org/) is a comprehensive approach to disable
colors by default for all tools:

All command-line software which outputs text with ANSI color added
should check for the presence of a NO_COLOR environment variable that,
when present (regardless of its value), prevents the addition of ANSI
color.

Signed-off-by: Leah Neukirchen <redacted>
---

This is a first stab at implementing NO_COLOR for git, effectively
making it then behave like before colors were enabled by default.

I feel this should be documented somewhere, but I'm not sure where the
best place is.  Perhaps in config.ui, or the Git environment variables
(but they all start with GIT_).

 color.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/color.c b/color.c
index d48dd947c..59e9c2459 100644
--- a/color.c
+++ b/color.c

@@ -326,6 +326,8 @@ int git_config_colorbool(const char *var, const char *value)
 
 static int check_auto_color(void)
 {
+	if (getenv("NO_COLOR"))
+		return 0;
 	if (color_stdout_is_tty < 0)
 		color_stdout_is_tty = isatty(1);
 	if (color_stdout_is_tty || (pager_in_use() && pager_use_color)) {

-- 
2.16.2