[vim] Make `GetGitBranchName` check for `fugitive`
[vim] Make `GetGitBranchName` check for `fugitive`

Change the `GetGitBranchName` function so that it only calls
`fugitive#head()` if the `fugitive` plugin is installed. This prevents
Vim from throwing an error when the `install_vim_plugins.sh` script is
executed for the first time.

AlbertoElias/dotfiles,girishramnani/dotfiles,wingy3181/dotfiles,AlbertoElias/dotfiles,codelandev/init-setup,b-boogaard/dotfiles,stevenirby/dotfiles-1,oddlord/dotfiles,zlot/dotfiles,sophatvathana/dotfiles,mickael83/ubuntu-dotfiles,girishramnani/dotfiles,makabde/dotfiles,wingy3181/dotfiles,zlot/dotfiles,EDiLD/dotfiles,khornberg/dotfiles,makabde/dotfiles,newtonne/dotfiles,wingy3181/dotfiles,mickael83/ubuntu-dotfiles,christophermoura/dotfiles,pbrooks/dotfiles,EDiLD/dotfiles,newtonne/dotfiles,christophermoura/dotfiles