git-diff: don't use colors when stdout is not a tty.

Currently, with diff-coloring enabled, git-diff outputs color escape sequences even if stdout is redirected (not a tty). This is rather annoying when trying to further process the output. This patch teaches the diff command to only do its coloring stuff when a pager is in use or stdout is a tty.

---
 diff.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index 1958970..988e741 100644
@@ -451,7 +451,7 @@ static void free_diff_words_data(struct emit_callback *ecbdata)
 
 const char *diff_get_color(int diff_use_color, enum color_diff ix)
 {
-       if (diff_use_color)
+       if (diff_use_color && (pager_in_use || isatty(STDOUT_FILENO)))
                return diff_colors[ix];
        return "";
 }

--
1.5.2.2

Set color.diff to auto instead of true.