Made git commands work despite values of GIT_DIR and GIT_WORK_TREE

[12qu]

The previous method for interacting with a bundle via git involved
cd'ing into the bundle directory then running the relevant git commands.
The problem with this approach is that if the environmental variables
GIT_DIR and GIT_WORK_TREE are set at this point then git will
preference their values over those implied by the current directory.
(This can cause trouble for vcsh users, for example.)

This patch uses the git arguments -C, --git-dir, and --work-tree
to specify the location of the bundle explicitly. This overcomes the
original problem since git gives these values precedence over GIT_DIR
and GIT_WORK_TREE.

[lucc]

The code looks good to me. I just ran a :PluginUpdate using this patch and it works well (does not seem to introduce errors). (I also set GIT_DIR and GIT_WORK_TREE to some nonexistent dirs for a test)

[jdevera]

This is exciting. By removing the && we gain some compatibility with the Fish shell, and we could at some point stop telling people to set their shells to bash in vim.

I am concerned about the -C option, I seem to recall reading about it in a git's changelog, which would mean it's relatively new. Must check this.

[lucc]

If you decide that we really can not use the -C option it can also be done with the --git-dir and --work-tree options alone.

[jdevera]

The -C option was introduced in git 1.8.5 in November 2013. I'd say it's long enough, and especially worth it considering it would rid us of the &&. @gmarik do you object?

[gmarik]

The -C option was introduced in git 1.8.5 in November 2013.

It's not that long ago.
This actually may cause a lot of problems for people.

[12qu]

@lucc Unfortunately I don't think this can be done with --git-dir and --work-tree alone, at least, not in a way that removes the &&. I tried the following code:

func! vundle#installer#expand_git_cmd(bundle, cmd) abort
  let git_dir = vundle#installer#shellesc(a:bundle.path().'/.git')
  let work_tree = vundle#installer#shellesc(a:bundle.path())
  let out = 'git --git-dir='.git_dir.' --work-tree='.work_tree.' '.a:cmd
  return out
endf

but receive an error fatal: /usr/lib/git-core/git-submodule cannot be used without a working tree. when the submodule update --init --recursive command is used. Apparently git submodule does not like --work-tree.

However, my original motivation for this patch was not to remove the && but rather to enable Vundle to work even when GIT_DIR and GIT_WORK_TREE are set. I still think there is reason to fix this, which may be done as follows:

func! vundle#installer#expand_git_cmd(bundle, cmd) abort
  let out = 'cd '.vundle#installer#shellesc(a:bundle.path()).' && git --git-dir=.git/ --work-tree=. '.a:cmd
  return out
endf

[jdevera]

Right on. The ability to remove '&&' was an observed nice side effect, doesn't matter if we don't get it.

[jdevera]

What if we, at the beginning of the installation, checked whether these vars are set, if so cache their values and unset them, and then at the end do the reverse? Seems to me like a simpler change.