[PATCH/RFC][GSoC] diff-no-index: transform "$directory $file" args to "$directory/$file $file"

[Yurii Shevtsov <ungetch@gmail.com>]

git diff --no-index refuses to compare if args are directory and file,
instead of usual diff.

Now git diff --no-index compares args' target names and, if they are
equal, refuses to compare. If args differ, it transforms args and
diffs files, as usual diff does.
This is the least expensive way in terms of amount of new code and
also doesn't affect usual behaviour.

Changes are done in queue_diff(). Adding args transformation logic to
top-level is irrational,
because requires writing additional code to define args' types, which
is already done in
queue_diff(). The error message about file\directory conflict left as
is until further guidence.

Signed-off-by: Yurii Shevtsov <ungetch@gmail.com>
---
 diff-no-index.c |   35 +++++++++++++++++++++++++++++++++--
 1 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/diff-no-index.c b/diff-no-index.c
index 265709b..9a3439a 100644
--- a/diff-no-index.c
+++ b/diff-no-index.c
@@ -97,8 +97,39 @@ static int queue_diff(struct diff_options *o,
     if (get_mode(name1, &mode1) || get_mode(name2, &mode2))
         return -1;

-    if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2))
-        return error("file/directory conflict: %s, %s", name1, name2);
+    if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2)) {
+        struct strbuf path;
+        const char *dir, *file;
+        char *filename, *dirname = 0;
+        int i, ret = 0;
+
+        dir = S_ISDIR(mode1) ? name1 : name2;
+        file = (dir == name1) ? name2 : name1;
+        strbuf_init(&path, strlen(name1) + strlen(name2) + 1);
+        strbuf_addstr(&path, dir);
+        filename = strrchr(file, '/');
+        if (path.len && path.buf[path.len - 1] != '/')
+            strbuf_addch(&path, '/');
+        for (i = path.len - 2; i >= 0; i--)
+            if (path.buf[i] == '/') {
+                dirname = &path.buf[i];
+                break;
+            }
+        if (dirname == 0)
+            dirname = path.buf;
+
+        if (!strncmp(dirname, filename, strlen(filename)))
+            return error("file/directory conflict: %s, %s", name1, name2);
+
+        strbuf_addstr(&path, filename ? (filename + 1) : file);
+        if (file == name1)
+            ret = queue_diff(o, file, path.buf);
+        else
+            ret = queue_diff(o, path.buf, file);
+        strbuf_release(&path);
+
+        return ret;
+    }

     if (S_ISDIR(mode1) || S_ISDIR(mode2)) {
         struct strbuf buffer1 = STRBUF_INIT;
--