Add 'merge' mode to 'git reset'

Add 'merge' mode to 'git reset'

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, just call it that: "git reset --merge".

I've wanted this for a long time, since I very commonly carry a dirty
tree while working on things. My main 'Makefile' file quite often has the
next version already modified, and sometimes I have local modifications
that I don't want to commit, but I still do pulls and patch applications,
and occasionally want to do "git reset" to undo them - while still keeping
my local modifications.

(Maybe we could eventually change it to something like "if we have a
working tree, default to --merge, otherwise default to --mixed").

NOTE! This new mode is certainly not perfect. There's a few things to look
out for:

 - if the index has unmerged entries, "--merge" will currently simply
   refuse to reset ("you need to resolve your current index first").
   You'll need to use "--hard" or similar in this case.

   This is sad, because normally a unmerged index means that the working
   tree file should have matched the source tree, so the correct action is
   likely to make --merge reset such a path to the target (like --hard),
   regardless of dirty state in-tree or in-index. But that's not how
   read-tree has ever worked, so..

 - "git checkout -m" actually knows how to do a three-way merge, rather
   than refuse to update the working tree. So we do know how to do that,
   and arguably that would be even nicer behavior.

   At the same time it's also arguably true that there is a chance of loss
   of state (ie you cannot get back to the original tree if the three-way
   merge ends up resolving cleanly to no diff at all), so the "refuse to
   do it" is in some respects the safer - but less user-friendly - option.

In other words, I think 'git reset --merge' could become a bit more
friendly, but this is already a big improvement. It allows you to undo a
recent commit without having to throw your current work away.

Yes, yes, with a dirty tree you could always do

	git stash
	git reset --hard
	git stash apply

instead, but isn't "git reset --merge" a nice way to handle one particular
simple case?

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
--

Hmm? Maybe I'm the only one that does a lot of work with a dirty tree, and
sure, I can do other things like the "git stash" thing, or using "git
checkout" to actually create a new branch, and then playing games with
branch renaming etc to make it work like this one.

But I suspect others dislike how "git reset" works too. But see the
suggested improvements above.

 builtin-reset.c |   26 ++++++++++++++++++--------
 1 files changed, 18 insertions(+), 8 deletions(-)