diff: support reading a file from stdin via "-"

This allows you to say

	echo Hello World | git diff x -

to compare the contents of file "x" with the line "Hello World".
This automatically switches to --no-index mode.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff-lib.c

@@ -37,14 +37,20 @@ static int queue_diff(struct diff_options *o,
 	int mode1 = 0, mode2 = 0;
 
 	if (name1) {
-		if (stat(name1, &st))
+		if (!strcmp(name1, "-"))
+			mode1 = 0644;
+		else if (stat(name1, &st))
 			return error("Could not access '%s'", name1);
-		mode1 = st.st_mode;
+		else
+			mode1 = st.st_mode;
 	}
 	if (name2) {
-		if (stat(name2, &st))
+		if (!strcmp(name2, "-"))
+			mode2 = 0644;
+		else if (stat(name2, &st))
 			return error("Could not access '%s'", name2);
-		mode2 = st.st_mode;
+		else
+			mode2 = st.st_mode;
 	}
 
 	if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2))
@@ -224,7 +230,7 @@ int setup_diff_no_index(struct rev_info *revs,
 {
 	int i;
 	for (i = 1; i < argc; i++)
-		if (argv[i][0] != '-')
+		if (argv[i][0] != '-' || argv[i][1] == '\0')
 			break;
 		else if (!strcmp(argv[i], "--")) {
 			i++;

diff.c

@@ -1389,6 +1389,22 @@ int diff_populate_filespec(struct diff_filespec *s, int size_only)
 		char *buf;
 		unsigned long size;
 
+		if (!strcmp(s->path, "-")) {
+#define INCREMENT 1024
+			int i = INCREMENT;
+			size = 0;
+			buf = NULL;
+			while (i == INCREMENT) {
+				buf = xrealloc(buf, size + INCREMENT);
+				i = xread(0, buf + size, INCREMENT);
+				size += i;
+			}
+			s->should_munmap = 0;
+			s->data = buf;
+			s->size = size;
+			s->should_free = 1;
+			return 0;
+		}
 		if (lstat(s->path, &st) < 0) {
 			if (errno == ENOENT) {
 			err_empty:
@@ -1689,6 +1705,10 @@ static void diff_fill_sha1_info(struct diff_filespec *one)
 	if (DIFF_FILE_VALID(one)) {
 		if (!one->sha1_valid) {
 			struct stat st;
+			if (!strcmp(one->path, "-")) {
+				hashcpy(one->sha1, null_sha1);
+				return;
+			}
 			if (lstat(one->path, &st) < 0)
 				die("stat %s", one->path);
 			if (index_path(one->sha1, one->path, &st, 0))