#86 Add support for nested VCS

[Anonymous]

I have my home directory version controlled with GIT. As such, vcs_info always shows that as being the current VCS, even when I'm in a subdirectory version controlled with SVN or P4.

Currently, the detection loop within vcs_info breaks on the first detected VCS. If instead the loop compiled a list of all detected VCS and their base directories, it could then select the inner most (determined by the longest path string) or the outer most (determined by the shortest path string) VCS based on the user's configuration (i.e. via zstyle).