git log: support "auto" decorations

This works kind of like "--color=auto" - add decorations for interactive
use, but do not change defaults when scripting or when piping the output
to anything but a terminal.

You can use either

    [log]
         decorate=auto

in the git config files, or the "--decorate=auto" command line option to
choose this behavior.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -63,6 +63,8 @@ static int parse_decoration_style(const char *var, const char *value)
 		return DECORATE_FULL_REFS;
 	else if (!strcmp(value, "short"))
 		return DECORATE_SHORT_REFS;
+	else if (!strcmp(value, "auto"))
+		return (isatty(1) || pager_in_use()) ? DECORATE_SHORT_REFS : 0;
 	return -1;
 }