cred_ssh_key() defaults (try 2)

[ijlyttle]

Replaces #316

Fixes #314

Adds a function ssh_path(), in the footsteps of @jennybc's git_user_dot_home()

Puts that function to use for the defaults in cred_ssh_key().

I don't see a test-file for credentials - I could add one if you think it useful.

• introduces ssh_path()
• integrates ssh_path() into cred_ssh_key() and ssh_key_needs_passphrase()

Signed-off-by: Ian J. Lyttle ian.lyttle@schneider-electric.com

[ijlyttle]

Transferring from #316:

@stewid I have installed afe2961 on both my Mac and PC, and I run:

git2r::push(".", "origin", "refs/heads/master", credentials = git2r::cred_ssh_key())

On the Mac, it seems to work OK.

On the PC, I get a strange error:

Error in is_detached(repo) : 
  Error in 'git2r_repository_head_detached': Invalid repository

The repo is otherwise normal (from the terminal, git status says all is well):

$ git status

On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working tree clean

[coveralls]

Coverage decreased (-0.09%) to 78.428% when pulling afe2961 on ijlyttle:cred_ssh_key-windows2 into ef3261a on ropensci:master.

[stewid]

I'll look at it later today

[stewid]

I have studied the code and I think the error is unrelated to the ssh_path function.