From: Luke Shumaker <redacted>

Don't silently ignore a flag that's invalid for a given subcommand.  The
user expected it to do something; we should tell the user that they are
mistaken, instead of surprising the user.

It could be argued that this change might break existing users.  I'd
argue that those existing users are already broken, and they just don't
know it.  Let them know that they're broken.

Signed-off-by: Luke Shumaker <redacted>
......

Thank you for the recent works, Luke.

I'd like to know if you are aware or working on the issue that branches
before "git subtree add --squash" are pushed via "git subtree push",
which has been reported by me at Tue, 16 Jul 2019 02:30:44 +0800, as I
haven't seen an obvious fix about it in the recent patches in a quick
glance.

Below is the original report (with a slight modification):

-----------------------------------------------------------------------

Say we have a repository with "master" and "sub" branches like below:

    A---B---C---D---E---F  master
               /
          X---Y

    X---Y  sub

where commits A and B include changes in subdirectory "sub", commit C
removes subdirectory "sub", commit D is generated by "git subtree add
-P sub Y", and commits E and F also include changes in subdirectory
"sub".

We'd get this if we run "git subtree push -P sub . sub" on master at F:

    X---Y---E'--F'  sub (after push)

On the other hand, if we have simliar trees except that commit D is
generated by "git subtree add --squash -P sub Y":

    A---B---C---D---E---F  master
               /
              Y'

    X---Y  sub

We'd expect to get this after we run "git subtree push -P sub . sub" on
master at F:

    X---Y---E'--F'  sub (after push, expected)

But actually we get this (in Git 2.22.0):

    A---B---C---D'--E'--F'  sub (after push, actual)
               /
          X---Y

This seems to be a side effect of 2.7.0 -> 2.7.1 in which a change is
made to include merged branches in "git subtree push", but mistakenly
causes branches before "git subtree add --squash" be included.



This bug is especially obvious if we simply run `git subtree push`
after a `git subtree add --squash`:

If master is at commit D, which is generated by
`git subtree add --squash -P sub Y`:

    A---B---C---D  master
               /
              Y'

    X---Y  sub

After running `git subtree push -P sub . sub`, we get:

    A---B---C---D'  sub (after push, actual)
               /
          X---Y

While the sub branch should not change at all!

This bug, seemingly introduced by 933cfeb90b5d03b4096db6d60494a6eedea25d03
critically makes --squash approach for `git subtree` almost unusable.