Add support for manipulating with Git credentials

With some services (as Bitbucket) the user is authenticated by email and password for https access to the repository. There is currently no way to do this in Weblate.

There is solution to use git credentials to handle these:

1. Configure ```git config --global credential.helper store```
2. Manage credentials in the `~/.git-credentials` file:

```
https://user%40example.com:password@bitbucket.org
```

Note that both username and password have to be urlencoded to avoid problems with special chars.