color: introduce support for colorizing stderr

So far, we only ever asked whether stdout wants to be colorful. In the
upcoming patches, we will want to make push errors more prominent, which
are printed to stderr, though.

So let's refactor the want_color() function into a want_color_fd()
function (which expects to be called with fd == 1 or fd == 2 for stdout
and stderr, respectively), and then define the macro `want_color()` to
use the want_color_fd() function.

And then also add a macro `want_color_stderr()`, for convenience and
for documentation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

color.c

@@ -319,18 +319,20 @@ int git_config_colorbool(const char *var, const char *value)
 	return GIT_COLOR_AUTO;
 }
 
-static int check_auto_color(void)
+static int check_auto_color(int fd)
 {
-	if (color_stdout_is_tty < 0)
-		color_stdout_is_tty = isatty(1);
-	if (color_stdout_is_tty || (pager_in_use() && pager_use_color)) {
+	static int color_stderr_is_tty = -1;
+	int *is_tty_p = fd == 1 ? &color_stdout_is_tty : &color_stderr_is_tty;
+	if (*is_tty_p < 0)
+		*is_tty_p = isatty(fd);
+	if (*is_tty_p || (fd == 1 && pager_in_use() && pager_use_color)) {
 		if (!is_terminal_dumb())
 			return 1;
 	}
 	return 0;
 }
 
-int want_color(int var)
+int want_color_fd(int fd, int var)
 {
 	/*
 	 * NEEDSWORK: This function is sometimes used from multiple threads, and
@@ -339,15 +341,15 @@ int want_color(int var)
 	 * is listed in .tsan-suppressions for the time being.
 	 */
 
-	static int want_auto = -1;
+	static int want_auto[3] = { -1, -1, -1 };
 
 	if (var < 0)
 		var = git_use_color_default;
 
 	if (var == GIT_COLOR_AUTO) {
-		if (want_auto < 0)
-			want_auto = check_auto_color();
-		return want_auto;
+		if (want_auto[fd] < 0)
+			want_auto[fd] = check_auto_color(fd);
+		return want_auto[fd];
 	}
 	return var;
 }

color.h

@@ -88,7 +88,9 @@ int git_config_colorbool(const char *var, const char *value);
  * Return a boolean whether to use color, where the argument 'var' is
  * one of GIT_COLOR_UNKNOWN, GIT_COLOR_NEVER, GIT_COLOR_ALWAYS, GIT_COLOR_AUTO.
  */
-int want_color(int var);
+int want_color_fd(int fd, int var);
+#define want_color(colorbool) want_color_fd(1, (colorbool))
+#define want_color_stderr(colorbool) want_color_fd(2, (colorbool))
 
 /*
  * Translate a Git color from 'value' into a string that the terminal can