Added test case to illustrate bogus conflicts detected if no files were merged, and untracked files exist in the workdir.
Only check for workdir conflicts if the index has merged files

Passing 0 as the length of the paths to check to git_diff_index_to_workdir
results in all files being treated as conflicting, that is, all untracked or
modified files in the worktree is reported as conflicting
Remove local unused index_repo variable
Fix typo
Fix attribute lookup in index for bare repos

When using a bare repo with an index, libgit2 attempts to read
files from the index.  It caches those files based on the path
to the file, specifically the path to the directory that contains
the file.

If there is no working directory, we use `git_path_dirname_r` to
get the path to the containing directory.  However, for the
`.gitattributes` file in the root of the repository, this ends up
normalizing the containing path to `"."` instead of the empty
string and the lookup the `.gitattributes` data fails.

This adds a test of attribute lookups on bare repos and also
fixes the problem by simply rewriting `"."` to be `""`.
Merge pull request #2566 from libgit2/rb/attr-with-bare

Fix attribute lookup in index for bare repos
attr: Cleanup the containing dir logic
Merge pull request #2561 from jacquesg/merge-skip

No files merged may result in bogus merge conflict error
Merge pull request #2565 from linquize/typo

Fix typo