diff-tree: add "verbose header" mode

This allows you to trivially do fancy and readable output. Something like

	git-rev-list HEAD | git-diff-tree -p -v --stdin kernel/ | less -S

gives a nice output of what has changed in the kernel/ subdirectory lately.

diff-tree.c

@@ -3,12 +3,14 @@
 #include "diff.h"
 
 static int silent = 0;
+static int verbose_header = 0;
 static int ignore_merges = 1;
 static int recursive = 0;
 static int read_stdin = 0;
 static int line_termination = '\n';
 static int generate_patch = 0;
 static const char *header = NULL;
+static const char *header_prefix = "";
 
 // What paths are we interested in?
 static int nr_paths = 0;
@@ -270,12 +272,59 @@ static int diff_tree_sha1(const unsigned char *old, const unsigned char *new, co
 	return retval;
 }
 
+static int get_one_line(const char *msg, unsigned long len)
+{
+	int ret = 0;
+
+	while (len--) {
+		ret++;
+		if (*msg++ == '\n')
+			break;
+	}
+	return ret;
+}
+
+static char *generate_header(const char *commit, const char *parent, const char *msg, unsigned long len)
+{
+	static char this_header[1000];
+	int offset;
+
+	offset = sprintf(this_header, "%s%s (from %s)\n", header_prefix, commit, parent);
+	if (verbose_header) {
+		int hdr = 1;
+
+		for (;;) {
+			const char *line = msg;
+			int linelen = get_one_line(msg, len);
+
+			if (!linelen)
+				break;
+			if (offset + linelen + 10 > sizeof(this_header))
+				break;
+
+			msg += linelen;
+			len -= linelen;
+			if (linelen == 1)
+				hdr = 0;
+			if (hdr)
+				continue;
+			memset(this_header + offset, ' ', 4);
+			memcpy(this_header + offset + 4, line, linelen);
+			offset += linelen + 4;
+		}
+		this_header[offset++] = '\n';
+		this_header[offset] = 0;
+	}
+
+	return this_header;
+}
+
 static int diff_tree_stdin(char *line)
 {
 	int len = strlen(line);
 	unsigned char commit[20], parent[20];
 	unsigned long size, offset;
-	static char this_header[100];
+	static char this_header[1000];
 	char *buf;
 
 	if (!len || line[len-1] != '\n')
@@ -305,9 +354,10 @@ static int diff_tree_stdin(char *line)
 	while (offset + 48 < size && !memcmp(buf + offset, "parent ", 7)) {
 		if (get_sha1_hex(buf + offset + 7, parent))
 			return -1;
-		sprintf(this_header, "%s (from %s)\n", line, sha1_to_hex(parent));
-		header = this_header;
+		header = generate_header(line, sha1_to_hex(parent), buf, size);
 		diff_tree_sha1(parent, commit, "");
+		if (!header && verbose_header)
+			header_prefix = "\ndiff-tree ";
 		offset += 48;
 	}
 	return -1;
@@ -354,6 +404,11 @@ int main(int argc, char **argv)
 			silent = 1;
 			continue;
 		}
+		if (!strcmp(arg, "-v")) {
+			verbose_header = 1;
+			header_prefix = "diff-tree ";
+			continue;
+		}
 		if (!strcmp(arg, "--stdin")) {
 			read_stdin = 1;
 			continue;