Rename git core commands to be "git-xxxx" to avoid name clashes.

This also regularizes the make. The source files themselves don't get
the "git-" prefix, because that's just inconvenient. So instead we just
make the rule that "git-xxxx" depends on "xxxx.c", and do that for
all the core programs (ie the old "git-mktag.c" got renamed to just
"mktag.c" to match everything else).

And "show-diff" got renamed to "git-diff-files" while at it, since
that's what it really should be to match the other git-diff-xxx cases.

mktag.c

@@ -0,0 +1,125 @@
+#include "cache.h"
+
+/*
+ * A signature file has a very simple fixed format: three lines
+ * of "object <sha1>" + "type <typename>" + "tag <tagname>",
+ * followed by some free-form signature that git itself doesn't
+ * care about, but that can be verified with gpg or similar.
+ *
+ * The first three lines are guaranteed to be at least 63 bytes:
+ * "object <sha1>\n" is 48 bytes, "type tag\n" at 9 bytes is the
+ * shortest possible type-line, and "tag .\n" at 6 bytes is the
+ * shortest single-character-tag line. 
+ *
+ * We also artificially limit the size of the full object to 8kB.
+ * Just because I'm a lazy bastard, and if you can't fit a signature
+ * in that size, you're doing something wrong.
+ */
+
+// Some random size
+#define MAXSIZE (8192)
+
+/*
+ * We refuse to tag something we can't verify. Just because.
+ */
+static int verify_object(unsigned char *sha1, const char *expected_type)
+{
+	int ret = -1;
+	unsigned long mapsize;
+	void *map = map_sha1_file(sha1, &mapsize);
+
+	if (map) {
+		char type[100];
+		unsigned long size;
+		void *buffer = unpack_sha1_file(map, mapsize, type, &size);
+
+		if (buffer) {
+			if (!strcmp(type, expected_type))
+				ret = check_sha1_signature(sha1, buffer, size, type);
+			free(buffer);
+		}
+		munmap(map, mapsize);
+	}
+	return ret;
+}
+
+static int verify_tag(char *buffer, unsigned long size)
+{
+	int typelen;
+	char type[20];
+	unsigned char sha1[20];
+	const char *object, *type_line, *tag_line;
+
+	if (size < 64 || size > MAXSIZE-1)
+		return -1;
+	buffer[size] = 0;
+
+	/* Verify object line */
+	object = buffer;
+	if (memcmp(object, "object ", 7))
+		return -1;
+	if (get_sha1_hex(object + 7, sha1))
+		return -1;
+
+	/* Verify type line */
+	type_line = object + 48;
+	if (memcmp(type_line - 1, "\ntype ", 6))
+		return -1;
+
+	/* Verify tag-line */
+	tag_line = strchr(type_line, '\n');
+	if (!tag_line)
+		return -1;
+	tag_line++;
+	if (memcmp(tag_line, "tag ", 4) || tag_line[4] == '\n')
+		return -1;
+
+	/* Get the actual type */
+	typelen = tag_line - type_line - strlen("type \n");
+	if (typelen >= sizeof(type))
+		return -1;
+	memcpy(type, type_line+5, typelen);
+	type[typelen] = 0;
+
+	/* Verify that the object matches */
+	if (get_sha1_hex(object + 7, sha1))
+		return -1;
+	if (verify_object(sha1, type))
+		return -1;
+
+	/* Verify the tag-name: we don't allow control characters or spaces in it */
+	tag_line += 4;
+	for (;;) {
+		unsigned char c = *tag_line++;
+		if (c == '\n')
+			break;
+		if (c > ' ')
+			continue;
+		return -1;
+	}
+
+	/* The actual stuff afterwards we don't care about.. */
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	unsigned long size;
+	char buffer[MAXSIZE];
+	unsigned char result_sha1[20];
+
+	if (argc != 1)
+		usage("cat <signaturefile> | git-mktag");
+
+	// Read the signature
+	size = read(0, buffer, MAXSIZE);
+
+	// Verify it for some basic sanity: it needs to start with "object <sha1>\ntype "
+	if (verify_tag(buffer, size) < 0)
+		die("invalid tag signature file");
+
+	if (write_sha1_file(buffer, size, "tag", result_sha1) < 0)
+		die("unable to write tag file");
+	printf("%s\n", sha1_to_hex(result_sha1));
+	return 0;
+}