Subject: Re: master^ is not a local branch -- huh?!?

From: Junio C Hamano <[email-masked]>
Date: Sat, 30 Jan 2010 11:02:09 -0800

Ron Garret <[email-masked]> writes:

>> > > If I do a git reset --hard then I get the old version, but I lose my 
>> > > HEAD pointer so that git diff doesn't give me what I want any more.

I think it would be helpful to point out one issue that may hurt you (and
anybody who thinks "git update --tree" without --index is a good idea).

Keeping the index and HEAD in sync but matching the work tree files to
that of a different commit has one major downside.  To git, a work tree
file that does not exist in the index is a yet-to-be-added-untracked file
(that is how and why "rm --cached file" works, for example).  It won't
participate in the diff output (other than being treated as "no such file
in the work tree version").

Suppose that you used to have a path in older revision, but not in the
current revision.  You remove everything from the work tree and replace it
with the files in the older revision, and you do not touch HEAD nor index.
"git diff -R" appears to show "what changed since that older version and
the latest", because it compares what is in the index relative to what is
in the work tree.  Nice.

Not quite.  Since the index does not know that path you recently removed,
you won't see that path.  If you run "git ls-files" for a list of files
known to git, it wouldn't be shown either.

Your original "git checkout master^" is a valid and probably the optimal
way to get a checkout of a older revision (which you could feed to your
running Lisp interpreter, in addition to being able to run "less" and
"make" on them).  Exactly because the index is updated to that of the
older version, you won't lose the sight of the path that you removed in a
later version, and you can review the change with "git diff -R master".

I think this is an XY problem that comes from your wanting to use "git
diff" (compare work tree with index) instead of "git diff $commit", and
that was because you wanted to use "HEAD" as a name of a commit.  If you
used a branch name you originally came from, none of this desire to "keep
index intact" or "keep HEAD intact" would have been necessary.

But this is all tangent; I think you now know more about git to improve
your IDE integration, without fighting with git but instead taking
advantage of it.