color: add color_set helper for copying raw colors

To set up default colors, we sometimes strcpy() from the
default string literals into our color buffers. This isn't a
bug (assuming the destination is COLOR_MAXLEN bytes), but
makes it harder to audit the code for problematic strcpy
calls.

Let's introduce a color_set which copies under the
assumption that there are COLOR_MAXLEN bytes in the
destination (of course you can call it on a smaller buffer,
so this isn't providing a huge amount of safety, but it's
more convenient than calling xsnprintf yourself).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -145,6 +145,11 @@ int color_parse(const char *value, char *dst)
 	return color_parse_mem(value, strlen(value), dst);
 }
 
+void color_set(char *dst, const char *color_bytes)
+{
+	xsnprintf(dst, COLOR_MAXLEN, "%s", color_bytes);
+}
+
 /*
  * Write the ANSI color codes for "c" to "out"; the string should
  * already have the ANSI escape code in it. "out" should have enough