Found via `codespell -i 3 -w --skip="./thirdparty,./share/locale,./fonts" -I ../musescore-whitelist.txt`
Whitelist:
```
clas
nd
ot
pres
strack
tage
te
```
This is or was the default, but for some reason we now sometimes see a
length of 9 on self-built versions, at least on Windows. So let's be
explict rather than relying on a default.
See https://musescore.org/en/node/152206#comment-699451
Make the post-checkout hook only update revision.h on branch-level
checkout. Add "assume unchanged" status to revision.h.
Expanded the description in README.
Added a default mscore/revision.h containing "Unknown" as the
revision number (mscore/revision.h remains in .gitignore).
Removed the dependency on "revision" in the Makefiles. Now
"make revision" must be done manually if required.
Added sample post-checkout git hook to allow git automatically
to update mscore/revision.h when doing a git checkout. If installed,
this removes the need to do "make revision" manually.
