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.

unpack-file.c

@@ -26,7 +26,7 @@ int main(int argc, char **argv)
 {
 	unsigned char sha1[20];
 
-	if (argc != 2 || get_sha1_hex(argv[1], sha1))
+	if (argc != 2 || get_sha1(argv[1], sha1))
 		usage("unpack-file.c <sha1>");
 
 	puts(create_temp_file(sha1));