Sub-package tag parsing breaks Windows path parsing in v2.1.0

[doug-fitzmaurice-rowden]

The feature to infer sub-package tags introduced in #161 and released in 2.1.0 breaks the default behaviour of the tool on Windows systems.

The features compares the root of the git repo to the current directory, and if they are not the same sets --pattern and --prefix defaults automatically to compensate.
However it appears that the git cli and the go os library return representations of the same path using different path separators, so the comparison always returns false, and the absolute path of the repo is set as the pattern.

The issue occurs both in the root of a git repo and in a subdirectory, and causes all tag parsing to fail. Normal behaviour can be restored by adding the old defaults back in on the CLI:

svu --pattern="" --prefix="v"

I think that using filepath.Clean() on both paths before using them should normalise the slashes and resolve this issue, though I can't test as I don't have a working golang setup on my Windows machine, and testing on Linux is inconclusive as filepath.Clean behaviour is platform-dependent.

Example:

> git rev-parse --show-toplevel
C:/Users/Doug/Code/myrepo

os.getWd()
C:\\Users\\Doug\\Code\\myrepo

This results in the following help text showing the defaults when running svu:

> svu -h
usage: svu [<flags>] <command> [<args> ...]

...snip....

      --pattern="C:\\Users\\Doug\\Code\\myrepo/*"
                                 limits calculations to be based on tags
                                 matching the given pattern
      --prefix="C:\\Users\\Doug\\Code\\myrepo/v"
                                 set a custom prefix

[caarlos0]

i don't use windows, happy to review a pr fixing this though