object: make git_object_lookup() return GIT_ENOTFOUND when searching for an existing object by specifying an incorrect type

'git commit' and 'git tag -a' enforce some conventions, like cleaning up excess whitespace and making sure that the last line ends with a '\n'. This fix replicates this behavior.

Fix libgit2/libgit2sharp#117

Signed-off-by: schu <schu-github@schulog.org>

in tag/write.c.

Also cleaned up some previously converted code that still had
little things to polish.

To conform the naming scheme of git_oid_fromstr we should change the
name of git_oid_to_string to git_oid_tostr.

Obviously all the old throw routines are still in place, so we can
gradually port over.

This adds some initial unit tests for file filtering and fixes
some simple bugs in filter application.

Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!