Merge branch 'lt/abbrev-auto'

Allow the default abbreviation length, which has historically been
7, to scale as the repository grows.  The logic suggests to use 12
hexdigits for the Linux kernel, and 9 to 10 for Git itself.

* lt/abbrev-auto:
  abbrev: auto size the default abbreviation
  abbrev: prepare for new world order
  abbrev: add FALLBACK_DEFAULT_ABBREV to prepare for auto sizing

diff.c

@@ -3468,7 +3468,7 @@ void diff_setup_done(struct diff_options *options)
 			 */
 			read_cache();
 	}
-	if (options->abbrev <= 0 || 40 < options->abbrev)
+	if (40 < options->abbrev)
 		options->abbrev = 40; /* full */
 
 	/*