Making git format-patch follow renames like git log --follow

Question

I am trying to move files from one local git repository to another local git repository for a different project while preserving history fr

Answer 1

Yuck. I think the problem is some broken logic. In particular, when you combine --reverse and --follow you must specify the old file name:

[rename foo to bar]
$ git log --follow bar  # works
$ git log --follow --reverse -- foo # requires "--" because foo is not in HEAD

This ... sort of works. Except, it then treats the file as deleted when it hits the rename, and everything stops there.

tree-diff.c contains this function:

static void try_to_follow_renames(...)
{
        ...
        /* Remove the file creation entry from the diff queue, and remember it */
        choice = q->queue[0];
        q->nr = 0;

which is called if diff_might_be_rename returns true:

static inline int diff_might_be_rename(void)
{
        return diff_queued_diff.nr == 1 &&
                !DIFF_FILE_VALID(diff_queued_diff.queue[0]->one);
}

...
int diff_tree_sha1(...)
{
        ...
        if (!*base && DIFF_OPT_TST(opt, FOLLOW_RENAMES) && diff_might_be_rename()) {

I'm making some large assumptions here, but when you go in the other order, instead of "file bar was just created, let's see if we can find a foo from which it was renamed", if the log is reversed you need to have "file foo deleted, let's see if we can find a bar to which it was renamed", and that's just ... missing, if the comment is accurate.

If you have a lot of these to do, I'd suggest attempting to add something here to remember if the diff is reversed (as it is for both format-patch and log --reverse) and change the diff_might_be_rename() and try_to_follow_renames() code as needed.

If you just have one, well, manually hacking up some diffs is probably easier. :-)