Introduce git_cred_ssh_interactive_new()

This allows for keyboard-interactive based SSH authentication
Make git_cred_ssh_custom_new() naming more consistent
Replace void * with proper callback types
Pop ignore only if whole relative path matches

When traversing the directory structure, the iterator pushes and
pops ignore files using a vector.  Some directories don't have
ignore files, so it uses a path comparison to decide when it is
right to actually pop the last ignore file.  This was only
comparing directory suffixes, though, so a subdirectory with the
same name as a parent could result in the parent's .gitignore
being popped off the list ignores too early.  This changes the
logic to compare the entire relative path of the ignore file.
Cleanup tests with helper functions
Only zero sensitive information on destruction (and memory actually allocated by us)
Preload attribute files that may contain macros

There was a latent bug where files that use macro definitions
could be parsed before the macro definitions were loaded.  Because
of attribute file caching, preloading files that are going to be
used doesn't add a significant amount of overhead, so let's always
preload any files that could contain macros before we assemble the
actual vector of files to scan for attributes.
Fix ignore difference from git with trailing /*

Ignore patterns that ended with a trailing '/*' were still needing
to match against another actual '/' character in the full path.
This is not the same behavior as core Git.

Instead, we strip a trailing '/*' off of any patterns that were
matching and just take it to imply the FNM_LEADING_DIR behavior.
Minor fixes

Only apply LEADING_DIR pattern munging to patterns in ignore and
attribute files, not to pathspecs used to select files to operate
on.  Also, allow internal macro definitions to be evaluated before
loading all external ones (important so that external ones can
make use of internal `binary` definition).
Merge pull request #2279 from libgit2/rb/moar-eegnöre-fîxés

Fix several ignore and attribute file behavior bugs
Merge pull request #2273 from jacquesg/ssh-interactive

Add support for SSH keyboard-interactive authentication