Re: [FIXED PATCH] Make rebase save ORIG_HEAD if changing current branch

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> The idea behind ORIG_HEAD is to have an anchoring point before an
> operation that moves your HEAD in a drastic way.  Think if it as a
> poor-man's reflog -- in fact it predates reflog.
>
> That is why reset saves away the HEAD before it does its thing, so that
> you can easily say "Oops, I did not mean it -- reset ORIG_HEAD" to flip
> back to the previous state.  Both a fast-forward merge and a real merge
> can be undone by resetting back to ORIG_HEAD.

I've also seen people complain (quite rightfully) that these FOO_HEAD
pseudo refs are not documented in a central place.

How about doing this?  It should make it clear what ORIG_HEAD is meant to
record, while describing others.

And to answer your "git rebase --onto this from that-branch" question, I
think ORIG_HEAD should record the tip of that-branch before rebase takes
place, not the commit you happened to be at before running it.  Switching
branch to that-branch is not the drastic and unforseeable part.  The
drastic and unforseeable change is rebasing and seeing that the rebased
result does not work with the new upstream `from`, and the user would want
to have a way to quickly rewind the tip of the branch back to the state
before the rebase.  The new paragraph added by this patch should hopefully
make this reasoning more clear.

-- >8 --
Documentation: update sections on naming revisions and revision ranges

Various *_HEAD pseudo refs were not documented in any central place.
Especially since we may be teaching rebase and am to record ORIG_HEAD,
it would be a good time to do so.

While at it, reword the explanation on r1..r2 notation to reduce
confusion.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

 Documentation/git-rev-parse.txt |   20 +++++++++++++++-----
 1 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 378a312..7184274 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -166,7 +166,7 @@ blobs contained in a commit.
   first match in the following rules:
 
   . if `$GIT_DIR/<name>` exists, that is what you mean (this is usually
-    useful only for `HEAD`, `FETCH_HEAD` and `MERGE_HEAD`);
+    useful only for `HEAD`, `FETCH_HEAD`, `ORIG_HEAD` and `MERGE_HEAD`);
 
   . otherwise, `$GIT_DIR/refs/<name>` if exists;
 
@@ -177,6 +177,16 @@ blobs contained in a commit.
   . otherwise, `$GIT_DIR/refs/remotes/<name>` if exists;
 
   . otherwise, `$GIT_DIR/refs/remotes/<name>/HEAD` if exists.
++
+HEAD names the commit your changes in the working tree is based on.
+FETCH_HEAD records the branch you fetched from a remote repository
+with your last 'git-fetch' invocation.
+ORIG_HEAD is created by commands that moves your HEAD in a drastic
+way, to record the position of the HEAD before their operation, so that
+you can change the tip of the branch back to the state before you ran
+them easily.
+MERGE_HEAD records the commit(s) you are merging into your branch
+when you run 'git-merge'.
 
 * A ref followed by the suffix '@' with a date specification
   enclosed in a brace
@@ -289,10 +299,10 @@ notation is used.  E.g. "`{caret}r1 r2`" means commits reachable
 from `r2` but exclude the ones reachable from `r1`.
 
 This set operation appears so often that there is a shorthand
-for it.  "`r1..r2`" is equivalent to "`{caret}r1 r2`".  It is
-the difference of two sets (subtract the set of commits
-reachable from `r1` from the set of commits reachable from
-`r2`).
+for it.  When you have two commits `r1` and `r2` (named according
+to the syntax explained in SPECIFYING REVISIONS above), you can ask
+for commits that are reachable from r2 but not from r1 by
+"`{caret}r1 r2`" and it can be written as "`r1..r2`".
 
 A similar notation "`r1\...r2`" is called symmetric difference
 of `r1` and `r2` and is defined as