Add "get_sha1()" helper function.

This allows the programs to use various simplified versions of
the SHA1 names, eg just say "HEAD" for the SHA1 pointed to by
the .git/HEAD file etc.

For example, this commit has been done with

	git-commit-tree $(git-write-tree) -p HEAD

instead of the traditional "$(cat .git/HEAD)" syntax.

sha1_file.c

@@ -45,6 +45,35 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 	return 0;
 }
 
+int get_sha1_file(const char *path, unsigned char *result)
+{
+	char buffer[60];
+	int fd = open(path, O_RDONLY);
+	int len;
+
+	if (fd < 0)
+		return -1;
+	len = read(fd, buffer, sizeof(buffer));
+	close(fd);
+	if (len < 40)
+		return -1;
+	return get_sha1_hex(buffer, result);
+}
+
+int get_sha1(const char *str, unsigned char *sha1)
+{
+	static char pathname[PATH_MAX];
+
+	if (!get_sha1_hex(str, sha1))
+		return 0;
+	if (!get_sha1_file(str, sha1))
+		return 0;
+	snprintf(pathname, sizeof(pathname), ".git/%s", str);
+	if (!get_sha1_file(pathname, sha1))
+		return 0;
+	return -1;
+}
+
 char * sha1_to_hex(const unsigned char *sha1)
 {
 	static char buffer[50];