diff: enable indent heuristic by default

The feature was included in v2.11 (released 2016-11-29) and we got no
negative feedback. Quite the opposite, all feedback we got was positive.

Turn it on by default. Users who dislike the feature can turn it off
by setting diff.indentHeuristic (which also configures plumbing commands,
see prior patches).

The change to t/t4051-diff-function-context.sh is needed because the
heuristic shifts the changed hunk in the patch.  To get the same result
regardless of the heuristic configuration, we modify the test file
differently:  We insert a completely new line after line 2, instead of
simply duplicating it.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index da96577..2c47ccb 100644
--- a/diff.c
+++ b/diff.c
@@ -27,7 +27,7 @@
 #endif
 
 static int diff_detect_rename_default;
-static int diff_indent_heuristic; /* experimental */
+static int diff_indent_heuristic = 1;
 static int diff_rename_limit_default = 400;
 static int diff_suppress_blank_empty;
 static int diff_use_color_default = -1;