Re: git-diff passes <rev>:<path> args to GIT_EXTERNAL_DIFF incorrectly?

[Junio C Hamano <gitster@pobox.com>]

Dmitry Nikulin <pastafariant@gmail.com> writes:

> $ env GIT_EXTERNAL_DIFF=./print_argv.py git diff
> origin/branch1:file1.txt origin/branch2:file2.txt

I didn't even know external-diff driver is called (and does not even segfaut)
in the "compare two blobs" hack codepath.

The syntax <tree-ish>:<path-in-the-tree> you have on the command
line resolves to a blob object name.  There is no leading directory
name, there is no permission bits or executable bit, there is no
filename, when "diff" is told to compare two blob objects this way.
THe "diff" machinery that drives the external (or internal for that
matter) diff will only get two 40-hex blob object names and nothing
else.  The only pieces of information you can trust among those the
external program may receive are the blob object name(s) and the
contents stored in the temporary files given to it.  The location of
these temporary files or their mode bits have no relation to the
"files" in some tree in the original repository, as that information
is long lost when you write <tree-ish>:<path-in-the-tree> to tell
Git to use that as a blob object name.

    $ git diff -M branch1 branch2 -- file1 file2

if file1 and file2 have similar-enough contents, may have a better
chance of what you wanted to ask Git (if I am guessing what it is,
that is).