[coreboot] How to update the repository and avoid dirty tree with submodules?

[Paul Menzel]

Dear coreboot folks,


Often I am running into problems, when updating the tree.

Normally I do `git pull --rebase` to update my branch, and rebase my
changes on top of the upstream master branch. But if something in the
submodules, under `3rdparty` changes, I get a dirty tree. The same is
true, when bisecting or checking out a very old commit.

Just today, after running `git pull --rebase`

```
$ git show
commit 3c645391c0b7272f66d842bed454c29ceb9ce0bf
Author: Marshall Dawson <marshalldawson3rd at gmail.com>
Date:   Sat Mar 25 17:49:45 2017 -0600

    3rdparty/blobs: Update for AMD Stoney Ridge
    
    Add the binaryPI file for the FT4 package and add SMU firmware to
    be consumed by fanless OPNs.
    
    Change-Id: I1c9b5ded6b494fac1553cc2ec7756a7a47386ecf
    Signed-off-by: Marshall Dawson <marshalldawson3rd at gmail.com>
    Reviewed-on: https://review.coreboot.org/18988
    Tested-by: build bot (Jenkins)
    Reviewed-by: Martin Roth <martinroth at google.com>

diff --git a/3rdparty/blobs b/3rdparty/blobs
index 8090bdd598..b5e64d7cf4 160000
--- a/3rdparty/blobs
+++ b/3rdparty/blobs
@@ -1 +1 @@
-Subproject commit 8090bdd59853599e469b7503ea473ca12e8c681b
+Subproject commit b5e64d7cf4984dcf8e284c1878d943e88d53de5d
$ git diff
diff --git a/3rdparty/blobs b/3rdparty/blobs
index b5e64d7cf4..8090bdd598 160000
--- a/3rdparty/blobs
+++ b/3rdparty/blobs
@@ -1 +1 @@
-Subproject commit b5e64d7cf4984dcf8e284c1878d943e88d53de5d
+Subproject commit 8090bdd59853599e469b7503ea473ca12e8c681b
```

In this situation, I am manually changing the directory under
`3rdparty/`, in this case `blobs`, and do `git reset --hard <hash>`
there to get a clean state.

Do you know of tricks that the submodules are, by default, always in
the state of the commits noted in the coreboot branch?

Do you have other hints or suggestions on how to work with git
submodules?


Thanks,

Paul
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