Unified rev-parse, with a revision object

Fetch fixes

Don't try to update anything if there are no heads to update. This
saves us from trying to look into a fetch refspec when there is none.

A better fix for compatibility with git when using remotes without
refspecs is still needed, but this stops us from segfaulting.

Keep the data around until free, as expected by our own fetch example

This will probably prevent many lookup/free
operations in calling code.

OpenBSD & sparc64 fixes

Mixing int and size_t through pointers leads to problems
in big-endian machines.

commit: correctly detect the start of the commit message

The end of the header is signaled by to consecutive LFs and the commit
message starts immediately after. Jumping over LFs at the start of the
message is a bug and leads to creating different commits if
when rebuilding history.

This also fixes an empty commit message being returned as "\n".

dec refcount on refdb instead of always freeing

Branch upstream configuration

Add functions to set and unset the upstream configuration to
complement the getter we already have.

Correctly return matched pathspec when passing "*" or "."

I also moved all tests related to notifying in their own file.

Reduce the number of unnecessary objects in pushed packs

Submodule diff tests and fixes

This adds tests for diffs with submodules in them and (perhaps
unsurprisingly) requires further fixes to be made.  Specifically,
this fixes:

- when considering if a submodule is dirty in the workdir, it was
  being treated as dirty even if only the index was dirty.
- git_diff_patch_to_str (and git_diff_patch_print) were "printing"
  the headers for files (and submodules) that were unmodified or
  had no meaningful content.
- added comment to previous fix and removed unneeded parens.

Fix submodule dirty states not showing if submodules comes before files, or there are only dirty submodules but no changed files

GIT_DIFF_PATCH_DIFFABLE was not set, so the diff content was not shown

When submodule is dirty, the hash may be the same, but the length is different because -dirty is appended

We can therefore compare the length or hash

Updated link to Haskell bindings

The old one hasn't been updated in a long time. This one is current.