range-diff: add filename to inner diff

In a range-diff it's not always clear which file a certain funcname of
the inner diff belongs to, because the diff header (or section header
as added in a previous commit) is not always visible in the
range-diff.

Add the filename to the inner diffs header, so it's always visible to
users.

This also allows us to add the filename + the funcname to the outer
diffs hunk headers using a custom userdiff pattern, which will be done
in the next commit.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
index c277756..d4de270 100755
--- a/t/t3206-range-diff.sh
+++ b/t/t3206-range-diff.sh
@@ -110,7 +110,7 @@
 	      14
 	4:  a63e992 ! 4:  d966c5c s/12/B/
 	    @@
-	     @@ A
+	     @@ file: A
 	      9
 	      10
 	    - B
@@ -169,7 +169,7 @@
 	      14
 	4:  a63e992 ! 4:  d966c5c s/12/B/
 	    @@
-	     @@ A
+	     @@ file: A
 	      9
 	      10
 	    - B
@@ -203,20 +203,24 @@
 	    Z    s/11/B/
 	    Z
 	    - ## file ##
+	    -@@ file: A
 	    + ## renamed-file ##
-	    Z@@ A
+	    +@@ renamed-file: A
 	    Z 8
 	    Z 9
+	    Z 10
 	4:  a63e992 ! 4:  1e6226b s/12/B/
 	    @@
 	    Z
 	    Z    s/12/B/
 	    Z
 	    - ## file ##
+	    -@@ file: A
 	    + ## renamed-file ##
-	    Z@@ A
+	    +@@ renamed-file: A
 	    Z 9
 	    Z 10
+	    Z B
 	EOF
 	test_cmp expected actual
 '
@@ -248,7 +252,7 @@
 	    +    s/11/B/ + remove file
 	    Z
 	    Z ## file ##
-	    Z@@ A
+	    Z@@ file: A
 	    @@
 	    Z 12
 	    Z 13
@@ -310,7 +314,7 @@
 	:      14<RESET>
 	:<RED>4:  d966c5c <RESET><YELLOW>!<RESET><GREEN> 4:  8add5f1<RESET><YELLOW> s/12/B/<RESET>
 	:    <REVERSE><CYAN>@@<RESET>
-	:    <CYAN> @@ A<RESET>
+	:    <CYAN> @@ file: A<RESET>
 	:      9<RESET>
 	:      10<RESET>
 	:    <REVERSE><RED>-<RESET><FAINT> BB<RESET>