Format git diff headers correctly when adding or removing files.

Copying the output of cgit and using it in patches now works when adding
files to or removing files from the repository. This is helpful for people
who use cgit in their patch workflow.

ui-diff.c

@@ -229,11 +229,6 @@ static void header(unsigned char *sha1, char *path1, int mode1,
 	html(" b/");
 	html_txt(path2);
 
-	if (is_null_sha1(sha1))
-		path1 = "dev/null";
-	if (is_null_sha1(sha2))
-		path2 = "dev/null";
-
 	if (mode1 == 0)
 		htmlf("<br/>new file mode %.6o", mode2);
 
@@ -251,13 +246,21 @@ static void header(unsigned char *sha1, char *path1, int mode1,
 			if (mode2 != mode1)
 				htmlf("..%.6o", mode2);
 		}
-		html("<br/>--- a/");
+		if (is_null_sha1(sha1)) {
+			path1 = "dev/null";
+			html("<br/>--- /");
+		} else
+			html("<br/>--- a/");
 		if (mode1 != 0)
 			cgit_tree_link(path1, NULL, NULL, ctx.qry.head,
 				       sha1_to_hex(old_rev_sha1), path1);
 		else
 			html_txt(path1);
-		html("<br/>+++ b/");
+		if (is_null_sha1(sha2)) {
+			path2 = "dev/null";
+			html("<br/>+++ /");
+		} else
+			html("<br/>+++ b/");
 		if (mode2 != 0)
 			cgit_tree_link(path2, NULL, NULL, ctx.qry.head,
 				       sha1_to_hex(new_rev_sha1), path2);

ui-patch.c

@@ -28,11 +28,6 @@ static void header(unsigned char *sha1, char *path1, int mode1,
 	subproject = (S_ISGITLINK(mode1) || S_ISGITLINK(mode2));
 	htmlf("diff --git a/%s b/%s\n", path1, path2);
 
-	if (is_null_sha1(sha1))
-		path1 = "dev/null";
-	if (is_null_sha1(sha2))
-		path2 = "dev/null";
-
 	if (mode1 == 0)
 		htmlf("new file mode %.6o\n", mode2);
 
@@ -50,8 +45,18 @@ static void header(unsigned char *sha1, char *path1, int mode1,
 			if (mode2 != mode1)
 				htmlf("..%.6o", mode2);
 		}
-		htmlf("\n--- a/%s\n", path1);
-		htmlf("+++ b/%s\n", path2);
+
+		if (is_null_sha1(sha1)) {
+			path1 = "dev/null";
+			htmlf("\n--- /%s\n", path1);
+		} else
+			htmlf("\n--- a/%s\n", path1);
+
+		if (is_null_sha1(sha2)) {
+			path2 = "dev/null";
+			htmlf("+++ /%s\n", path2);
+		} else
+			htmlf("+++ b/%s\n", path2);
 	}
 }