[PATCH v5 0/3] completion of 'git [-C <dir>] diff'

The primary motivation for this topic is that the command-line completion of 'git diff' does not handle paths (unlike 'git status' and 'git add') and instead relies on the default behavior of Bash command-line completion, which completes files in $PWD; this does not work at all with the '-C <directory>' option.

This series teaches the completion machinery to complete revisions (unless '--' exists), then tracked paths, and then untracked paths, before letting the Bash default kick in. This way, we correctly complete 'git diff' command line even when '-C <directory>' is in effect.

The v5 iteration addresses two points identified by Elijah in v4. Hopefully this will be the "small and final" reroll.

 1/3: completion: no-op refactoring of diff completion
 2/3: completion: complete tracked paths for 'git diff'
 3/3: completion: 'git diff' completes untracked paths as a last
        resort

 contrib/completion/git-completion.bash | 69 +++++++++++++++-----------
 t/t9902-completion.sh                  | 59 ++++++++++++++++++++++
 2 files changed, 100 insertions(+), 28 deletions(-)

Range-diff against v4:
1:  d3c51c042c ! 1:  8295035d13 completion: no-op refactoring of diff completion
    @@ contrib/completion/git-completion.bash: __git_diff_difftool_options="--cached --
     -	--*)
     -		__gitcomp "$__git_diff_difftool_options"
     -		return
    +-		;;
    +-	esac
    +-	__git_complete_revlist_file
     +	if ! __git_has_doubledash; then
     +		case "$cur" in
     +		--diff-algorithm=*)
     +			__gitcomp "$__git_diff_algorithms" \
     +				"" "${cur##--diff-algorithm=}"
     +			return
    - 		;;
    --	esac
    --	__git_complete_revlist_file
    ++			;;
     +		--submodule=*)
     +			__gitcomp "$__git_diff_submodule_formats" \
     +				"" "${cur##--submodule=}"
2:  c3658d6ca2 ! 2:  dbb14298c1 completion: complete tracked paths for 'git diff'
    @@ contrib/completion/git-completion.bash: _git_diff ()
      	fi
     +
     +	if [ ${#COMPREPLY[@]} -eq 0 ]; then
    -+		__git_complete_index_file
    ++		__git_complete_index_file ""
     +	fi
      }
      
3:  ba5dc6f164 ! 3:  d661a1a5dd completion: 'git diff' completes untracked paths as a last resort
    @@ Commit message
      ## contrib/completion/git-completion.bash ##
     @@ contrib/completion/git-completion.bash: _git_diff ()
      	if [ ${#COMPREPLY[@]} -eq 0 ]; then
    - 		__git_complete_index_file
    + 		__git_complete_index_file ""
      	fi
     +
     +	if [ ${#COMPREPLY[@]} -eq 0 ]; then