git-completion.bash: add support for path completion

git-completion.bash: add support for path completion

The git-completion.bash script did not implemented full, git aware,
support to complete paths, for git commands that operate on files within
the current working directory or the index.

As an example:

	git add <TAB>

will suggest all files in the current working directory, including
ignored files and files that have not been modified.

Support path completion, for git commands where the non-option arguments
always refer to paths within the current working directory or the index,
as follows:

* the path completion for the "git rm" and "git ls-files"
  commands will suggest all cached files.

* the path completion for the "git add" command will suggest all
  untracked and modified files.  Ignored files are excluded.

* the path completion for the "git clean" command will suggest all
  untracked files.  Ignored files are excluded.

* the path completion for the "git mv" command will suggest all cached
  files when expanding the first argument, and all untracked and cached
  files for subsequent arguments.  In the latter case, empty directories
  are included and ignored files are excluded.

* the path completion for the "git commit" command will suggest all
  files that have been modified from the HEAD, if HEAD exists, otherwise
  it will suggest all cached files.

For all affected commands, completion will always stop at directory
boundary.  Only standard ignored files are excluded, using the
--exclude-standard option of the ls-files command.

When using a recent Bash version, Git path completion will be the same
as builtin file completion, e.g.

	git add contrib/

will suggest relative file names.

Signed-off-by: Manlio Perillo <manlio.perillo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>