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>

Documentation/config.txt

@@ -783,10 +783,11 @@ core.sparseCheckout::
 	linkgit:git-read-tree[1] for more information.
 
 core.abbrev::
-	Set the length object names are abbreviated to.  If unspecified,
-	many commands abbreviate to 7 hexdigits, which may not be enough
-	for abbreviated object names to stay unique for sufficiently long
-	time.
+	Set the length object names are abbreviated to.  If
+	unspecified or set to "auto", an appropriate value is
+	computed based on the approximate number of packed objects
+	in your repository, which hopefully is enough for
+	abbreviated object names to stay unique for some time.
 
 add.ignoreErrors::
 add.ignore-errors (deprecated)::

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;
 	}