config.abbrev: document the new default that auto-scales

We somehow forgot to update the "default is 7" in the
documentation.  Also give a way to explicitly ask the auto-scaling
by setting config.abbrev to "auto".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/config.c b/config.c
index 83fdecb..1c57120 100644
--- a/config.c
+++ b/config.c
@@ -834,10 +834,16 @@ static int git_default_core_config(const char *var, const char *value)
 	}
 
 	if (!strcmp(var, "core.abbrev")) {
-		int abbrev = git_config_int(var, value);
-		if (abbrev < minimum_abbrev || abbrev > 40)
-			return -1;
-		default_abbrev = abbrev;
+		if (!value)
+			return config_error_nonbool(var);
+		if (!strcasecmp(value, "auto"))
+			default_abbrev = -1;
+		else {
+			int abbrev = git_config_int(var, value);
+			if (abbrev < minimum_abbrev || abbrev > 40)
+				return error("abbrev length out of range: %d", abbrev);
+			default_abbrev = abbrev;
+		}
 		return 0;
 	}