log: add "log.follow" configuration variable

People who work on projects with mostly linear history with frequent
whole file renames may want to always use "git log --follow" when
inspecting the life of the content that live in a single path.

Teach the command to behave as if "--follow" was given from the
command line when log.follow configuration variable is set *and*
there is one (and only one) path on the command line.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.h b/diff.h
index c7ad42a..f7208ad 100644
--- a/diff.h
+++ b/diff.h
@@ -91,6 +91,7 @@
 #define DIFF_OPT_DIRSTAT_BY_LINE     (1 << 28)
 #define DIFF_OPT_FUNCCONTEXT         (1 << 29)
 #define DIFF_OPT_PICKAXE_IGNORE_CASE (1 << 30)
+#define DIFF_OPT_DEFAULT_FOLLOW_RENAMES (1 << 31)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_TOUCHED(opts, flag)    ((opts)->touched_flags & DIFF_OPT_##flag)