Convert gitdir paths to posix on Windows

Apparently, a .git file with "gitdir: path" link on Windows is
allowed to use backslashes in the path.  Who knew?
Merge pull request #2058 from libgit2/rb/gitdir-windows-paths

Convert gitdir paths to posix on Windows
Fix segfault when calling git_config_get_* functions when a config fails to load

Reinitialize the result code of get_entry() to GIT_ENOTFOUND
Merge pull request #2059 from linquize/git_config_get_crash

Fix segfault when calling git_config_get_* functions when a config fails to load