Add providing location for fetch_via_{vcs,git}

[aalexanderr]

This allows to call fetch_via_{vcs,git} multiple times for a location to
have another revision.

Example:

fetch_via_git("git+https://github.com/nexB/fetchcode.git", location="/tmp/repo")

will checkout tip of default branch

fetch_via_git("git+https://github.com/nexB/fetchcode.git@ccb7b6199681910ccf047f1a18aa89ece45d665c",
     location="/tmp/repo")

will reset to ccb7b61

Additionally:

• removed some trailing whitespaces
• used psf/black for code formatting (latest release, default values, let me know if I should've configured it not to switch ''->"" or sth other or if those commits should be dropped at all.
• parametrize test functions to lessen code repetition

[TG1999]

Hey, @aalexanderr thanks for your contribution, please can you also add some tests ? @pombredanne please can you chime in :)

[TG1999]

And please consider using black for formatting of code, https://github.com/psf/black

[aalexanderr]

Hi, glad to contribute!
I'll work on adding tests for this tomorrow/monday.

Will also need to handle some cases like:
fetch_via_git("git+https://github.com/nexB/fetchcode.git", location="/tmp/repo")
fetch_via_git("git+https://github.com/nexB/scancode-toolkit.git", location="/tmp/repo")

[pombredanne]

@TG1999 do you know if there is a way to get a shallow clone too?

[aalexanderr]

Hi, wanted to update -
When I started writing tests for stuff from pip.vcs I thought I'll better fix some stuff first:

• no attribution to pip's code authors
• as some pip.vcs code was commented out & due to @pombredanne question about shallow clone I decided to not move to pip as git submodule/subtree but rather clean it up for specialization needed in fetchcode
• requirements.txt are updated & contain versions of pkgs to be used but are not actually used
• lots of completely unused code (that doesn't seem relevant to me from this pkg point of view - let me know if I removed too much)

I'll add some tests in the near future but as I want it to be able to cover a bit more than is in scope of this PR I'll need a bit more time to work on it (as in mocks, fixtures etc)

As for this PR I'll probably split it too:

• applying formatting on all fetchcode via black
• vcs test parametrization (fyi- some test functions had same names, after this PR is applied the test no increases due to that)
• the actual change from the title

[aalexanderr]

@TG1999 do you know if there is a way to get a shallow clone too?

we might add it but it would need to be properly documented as some of the features of shallow clone are not yet done in git
https://git-scm.com/docs/shallow
thus we might also warn users that e.g. getting tags from history in case of shallow clone might not be possible (yet) + possible other stuff from previous versions of git

[aalexanderr]

regarding shallow clone, this might be better (already implemented in pip's vcs) https://git-scm.com/docs/partial-clone