git diff <file> HEAD^:<file> error message

["Carlos Martín Nieto" <cmn@elego.de>]

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Hello,

I was trying to figure out why running

   git diff HEAD^:RelNotes RelNotes

gives the expected output (on maint it tells me that the stable
version changed from 1.7.8.3 to 1.7.8.4) but swapping the arguments
doesn't.

   git diff RelNotes HEAD^:RelNotes

doesn't show the opposite patch but tells me that RelNotes doesn't
exist in HEAD^ which is clearly a lie (it sounds like it's a
misunderstanding on git's part, but it's certainly not the truth). In
both cases, verify_filename gets called and tries to stat both
HEAD^:RelNotes and RelNotes. In the "bad" (latter) case, after it
fails to find a file named "HEAD^:RelNotes" it ends up calling
diagnose_invalid_sha1_path which is not correct according to the
command just before the function

    /* Must be called only when object_name:filename doesn't exist. */

It looks like get_sha1_with_context_1 gets confused because we pass it
a filename which looks like object_name:filename even though we
earlier parsed it simply as a filename which happens to have a colon
inside it.

Another issue is that I'm not sure that the error message should even
get shown. The documentation tells me that I should be able to compare
two random blobs, though this mode doesn't seem to work if the first
argument is a file. I realise that a file isn't a blob, but since
specifying the arguments the other way around (blob, file) does work,
it looks to me like it should as well.

   cmn

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