There is one well-known and well-tested parser which we should use,
instead of implementing parsing a second time.

The common parser is also augmented to copy the LHS into the RHS if the
latter is empty.

The expressions test had to change a bit, as we now catch a bad RHS of a
refspec locally.

ignore: consider files with a CR in their names

We currently consider CR to start the end of the line, but that means
that we miss cases with CR CR LF which can be used with git to match
files whose names have CR at the end of their names.

The fix from the patch comes from Russell's comment in the issue.

This fixes #2536.

Make config reading continue after hitting a missing include file.

* Error-handling is cleaned up to only let a file-not-found error
  through, not other sorts of errors.  And when a file-not-found
  error happens, we clean up the error.
* Test now checks that file-not-found introduces no error.  And
  other minor cleanups.

An anonymous remote wouldn't create remote-tracking branches, so testing
we don't create them for TAGS_ALL is nonsensical. Furthermore, the name
of the supposed remote-tracking branch was also not one which would have
been created had it had a name.

Give the remote a name and test that we only create the tags when we
pass TAGS_ALL and that we do create the remote-branch branch when given
TAGS_AUTO.

For example, if you have

[include]
path = foo

and foo didn't exist, git_config_open_ondisk() would just give up
on the rest of the file.  Now it ignores the unresolved include
without error and continues reading the rest of the file.

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.