difftool: display the number of files in the diff queue in the prompt

When --prompt option is set, git-difftool displays a prompt for each
modified file to be viewed in an external diff program.  At that
point, it could be useful to display a counter and the total number
of files in the diff queue.

Below is the current difftool prompt for the first of 5 modified files:

    Viewing: 'diff.c'
    Launch 'vimdiff' [Y/n]:

Consider the modified prompt:

    Viewing (1/5): 'diff.c'
    Launch 'vimdiff' [Y/n]:

The current GIT_EXTERNAL_DIFF mechanism does not tell the number of
paths in the diff queue nor the current counter.  To make this
"counter/total" info available for GIT_EXTERNAL_DIFF programs
without breaking existing ones by doing the following:

 - Keep track of the number of paths shown so far in diff_options;

 - Export two new environment variables from run_external_diff() to
   show the total number of paths (from diff_queue_struct) and the
   current value of the counter (from diff_options); and

 - Update git-difftool--helper to use these two environment variables.

Signed-off-by: Zoltan Klinger <zoltan.klinger@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4020-diff-external.sh

@@ -193,6 +193,19 @@ test_expect_success 'GIT_EXTERNAL_DIFF with more than one changed files' '
 	GIT_EXTERNAL_DIFF=echo git diff
 '
 
+test_expect_success 'GIT_EXTERNAL_DIFF path counter/total' '
+	write_script external-diff.sh <<-\EOF &&
+	echo $GIT_DIFF_PATH_COUNTER of $GIT_DIFF_PATH_TOTAL >>counter.txt
+	EOF
+	>counter.txt &&
+	cat >expect <<-\EOF &&
+	1 of 2
+	2 of 2
+	EOF
+	GIT_EXTERNAL_DIFF=./external-diff.sh git diff &&
+	test_cmp expect counter.txt
+'
+
 test_expect_success 'GIT_EXTERNAL_DIFF generates pretty paths' '
 	touch file.ext &&
 	git add file.ext &&