Linus Torvalds authored Dec 01, 2008 and Junio C Hamano committed Dec 02, 2008

We have always had a nice way to reset a working tree to another state
while carrying our changes around: "git read-tree -u -m". Yes, it fails if
the target tree is different in the paths that are dirty in the working
tree, but this is how we used to switch branches in "git checkout", and it
worked fine.

However, perhaps exactly _because_ we've supported this from very early
on, another low-level command, namely "git reset", never did.

But as time went on, 'git reset' remains as a very common command, while
'git read-tree' is now a very odd and low-level plumbing thing that nobody
sane should ever use, because it only makes sense together with other
operations like either switching branches or just rewriting HEAD.

Which means that we have effectively lost the ability to do something very
common: jump to another point in time without always dropping all our
dirty state.

So add this kind of mode to "git reset", and since it merges your changes
to what you are resetting to...