range-diff: indent special lines as context

The range-diff coloring is a bit fuzzy when it comes to special lines of
a diff, such as indicating new and old files with +++ and ---, as it
would pickup the first character and interpret it for its coloring, which
seems annoying as in regular diffs, these lines are colored bold via
DIFF_METAINFO.

By indenting these lines by a white space, they will be treated as context
which is much more useful, an example [1] on the range diff series itself:

[...]
    + diff --git a/Documentation/git-range-diff.txt b/Documentation/git-range-diff.txt
    + new file mode 100644
    + --- /dev/null
    + +++ b/Documentation/git-range-diff.txt
    +@@
    ++git-range-diff(1)
[...]
    +
      diff --git a/Makefile b/Makefile
      --- a/Makefile
      +++ b/Makefile
[...]

The first lines that introduce the new file for the man page will have the
'+' sign colored and the rest of the line will be bold.

The later lines that indicate a change to the Makefile will be treated as
context both in the outer and inner diff, such that those lines stay
regular color.

[1] ./git-range-diff pr-1/dscho/branch-diff-v3...pr-1/dscho/branch-diff-v4
    These tags are found at https://github.com/gitgitgadget/git

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -90,6 +90,7 @@ static int read_patches(const char *range, struct string_list *list)
 			strbuf_addch(&buf, '\n');
 			if (!util->diff_offset)
 				util->diff_offset = buf.len;
+			strbuf_addch(&buf, ' ');
 			strbuf_addbuf(&buf, &line);
 		} else if (in_header) {
 			if (starts_with(line.buf, "Author: ")) {
@@ -126,6 +127,7 @@ static int read_patches(const char *range, struct string_list *list)
 			strbuf_addch(&buf, ' ');
 			strbuf_add(&buf, line.buf + 1, line.len - 1);
 		} else {
+			strbuf_addch(&buf, ' ');
 			strbuf_addbuf(&buf, &line);
 		}
 

t/t3206-range-diff.sh

@@ -133,9 +133,9 @@ test_expect_success 'changed message' '
 	    Z
 	    +    Also a silly comment here!
 	    +
-	    Zdiff --git a/file b/file
-	    Z--- a/file
-	    Z+++ b/file
+	    Z diff --git a/file b/file
+	    Z --- a/file
+	    Z +++ b/file
 	3:  147e64e = 3:  b9cb956 s/11/B/
 	4:  a63e992 = 4:  8add5f1 s/12/B/
 	EOF
@@ -152,9 +152,9 @@ test_expect_success 'dual-coloring' '
 	:     <RESET>
 	:    <REVERSE><GREEN>+<RESET><BOLD>    Also a silly comment here!<RESET>
 	:    <REVERSE><GREEN>+<RESET>
-	:     diff --git a/file b/file<RESET>
-	:    <RED> --- a/file<RESET>
-	:    <GREEN> +++ b/file<RESET>
+	:      diff --git a/file b/file<RESET>
+	:      --- a/file<RESET>
+	:      +++ b/file<RESET>
 	:<RED>3:  0559556 <RESET><YELLOW>!<RESET><GREEN> 3:  b9cb956<RESET><YELLOW> s/11/B/<RESET>
 	:    <REVERSE><CYAN>@@ -10,7 +10,7 @@<RESET>
 	:      9<RESET>