unpack-trees: allow Porcelain to give different error messages

The plumbing output is sacred as it is an API.  We _could_ change it if it
is broken in such a way that it cannot convey necessary information fully,
but we just do not _reword_ for the sake of rewording.  If somebody does
not like it, s/he is complaining too late.  S/he should have been here in
early May 2005 and make the language used by the API closer to what humans
read.  S/he wasn't here.  Too bad, and it is too late.

And people who complain should look at a bigger picture.  Look at what was
suggested by one of them and think for five seconds:

     $ git checkout mytopic
    -fatal: Entry 'frotz' not uptodate. Cannot merge.
    +fatal: Entry 'frotz' has local changes. Cannot merge.

If you do not see something wrong with this output, your brain has already
been rotten with use of git for too long a time.  Nobody asked us to
"merge" but why are we talking about "Cannot merge"?

This patch introduces a mechanism to allow Porcelains to specify messages
that are different from the ones that is given by the underlying plumbing
implementation of read-tree, so that we can reword the message Porcelains give
without disrupting the output from the plumbing.

    $ git-checkout pu
    error: You have local changes to 'Makefile'; cannot switch branches.

There are other places that ask unpack_trees() to n-way merge, detect
issues  and let it issue error message on its own, but I did this as a
demonstration and replaced only one message.

Yes I know about C99 structure initializers.  I'd love to use them but we
try to be nice to compilers without it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/unpack-trees.h b/unpack-trees.h
index d436d6ced..94e567265 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -8,6 +8,14 @@ struct unpack_trees_options;
 typedef int (*merge_fn_t)(struct cache_entry **src,
 		struct unpack_trees_options *options);
 
+struct unpack_trees_error_msgs {
+	const char *would_overwrite;
+	const char *not_uptodate_file;
+	const char *not_uptodate_dir;
+	const char *would_lose_untracked;
+	const char *bind_overlap;
+};
+
 struct unpack_trees_options {
 	unsigned int reset:1,
 		     merge:1,
@@ -23,6 +31,7 @@ struct unpack_trees_options {
 	int pos;
 	struct dir_struct *dir;
 	merge_fn_t fn;
+	struct unpack_trees_error_msgs msgs;
 
 	int head_idx;
 	int merge_size;