Remove src/or/or_sha1.i and src/common/common_sha1.i on `make clean` and remove
the temporary micro-revision file when its no longer needed.

Additional changes;
- show a message when generating the micro-revision file.
- add the temporary micro revision file to the list of files to be removed on
  `make clean` just in case.
- fix indentation of the make rule to improve readability.