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>

Documentation/git.txt

@@ -804,6 +804,15 @@ temporary file --- it is removed when 'GIT_EXTERNAL_DIFF' exits.
 +
 For a path that is unmerged, 'GIT_EXTERNAL_DIFF' is called with 1
 parameter, <path>.
++
+For each path 'GIT_EXTERNAL_DIFF' is called, two environment variables,
+'GIT_DIFF_PATH_COUNTER' and 'GIT_DIFF_PATH_TOTAL' are set.
+
+'GIT_DIFF_PATH_COUNTER'::
+	A 1-based counter incremented by one for every path.
+
+'GIT_DIFF_PATH_TOTAL'::
+	The total number of paths.
 
 other
 ~~~~~

diff.c

@@ -2899,11 +2899,16 @@ static void run_external_diff(const char *pgm,
 			      struct diff_filespec *one,
 			      struct diff_filespec *two,
 			      const char *xfrm_msg,
-			      int complete_rewrite)
+			      int complete_rewrite,
+			      struct diff_options *o)
 {
 	const char *spawn_arg[10];
 	int retval;
 	const char **arg = &spawn_arg[0];
+	struct diff_queue_struct *q = &diff_queued_diff;
+	const char *env[3] = { NULL };
+	char env_counter[50];
+	char env_total[50];
 
 	if (one && two) {
 		struct diff_tempfile *temp_one, *temp_two;
@@ -2928,7 +2933,14 @@ static void run_external_diff(const char *pgm,
 	}
 	*arg = NULL;
 	fflush(NULL);
-	retval = run_command_v_opt(spawn_arg, RUN_USING_SHELL);
+
+	env[0] = env_counter;
+	snprintf(env_counter, sizeof(env_counter), "GIT_DIFF_PATH_COUNTER=%d",
+			++o->diff_path_counter);
+	env[1] = env_total;
+	snprintf(env_total, sizeof(env_total), "GIT_DIFF_PATH_TOTAL=%d", q->nr);
+
+	retval = run_command_v_opt_cd_env(spawn_arg, RUN_USING_SHELL, NULL, env);
 	remove_tempfile();
 	if (retval) {
 		fprintf(stderr, "external diff died, stopping at %s.\n", name);
@@ -3042,7 +3054,7 @@ static void run_diff_cmd(const char *pgm,
 
 	if (pgm) {
 		run_external_diff(pgm, name, other, one, two, xfrm_msg,
-				  complete_rewrite);
+				  complete_rewrite, o);
 		return;
 	}
 	if (one && two)
@@ -3317,6 +3329,8 @@ void diff_setup_done(struct diff_options *options)
 		options->output_format = DIFF_FORMAT_NO_OUTPUT;
 		DIFF_OPT_SET(options, EXIT_WITH_STATUS);
 	}
+
+    options->diff_path_counter = 0;
 }
 
 static int opt_arg(const char *arg, int arg_short, const char *arg_long, int *val)

diff.h

@@ -164,6 +164,8 @@ struct diff_options {
 	diff_prefix_fn_t output_prefix;
 	int output_prefix_length;
 	void *output_prefix_data;
+
+	int diff_path_counter;
 };
 
 enum color_diff {

git-difftool--helper.sh

@@ -40,7 +40,8 @@ launch_merge_tool () {
 	# the user with the real $MERGED name before launching $merge_tool.
 	if should_prompt
 	then
-		printf "\nViewing: '%s'\n" "$MERGED"
+		printf "\nViewing (%s/%s): '%s'\n" "$GIT_DIFF_PATH_COUNTER" \
+			"$GIT_DIFF_PATH_TOTAL" "$MERGED"
 		if use_ext_cmd
 		then
 			printf "Launch '%s' [Y/n]: " \

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 &&