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>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/git.txt b/Documentation/git.txt
index b73a24a..25b9002 100644
--- a/Documentation/git.txt
+++ b/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
 ~~~~~