Mercurial: Any way to "merge and discard changes"? - mercurial

Question

I have a graphlog that looks something like this:
(snip)
| |
| o 1) Other Dev: Commit
| | \
o | | 2) Me: Commit
/ | | |
| | o | 3) Other Dev: Commits with an error
| |/| |
| o |/ 4) Me: Merge and commit
| /|
|/ |
o | 5) Me: Realize there were bugs in the commit and take earlier version to merge with
| o 6) Other Dev: Fixes error
o / 7) Me: committing some changes
|/
o 8) Me: Merge fixed tip
At (8), everything is as it should be with the exception of the dangling extra head at (4). To get rid of it I have to merge (4) -.-> (8) but, since there is nothing in (4) that I need, I can safely discard all of it's changes. I could do this merge manually file-by-file (and usually this isn't that big a deal) but for my own edification - is there a simple one-line way to say "merge (4) with (8) and always take (8)"?

Answer 1

Yes. The internal:local builtin merge tool.
$ hg merge 4 --tool internal:local
Similarly there's internal:other that picks the other version of files as the merged version.
Here's an example to clarify what's going on, start off with a repo with a single file:
$ echo a >> a
$ hg ci -Am.
adding a
$ echo a >> a
$ hg ci -Am.
Branch it and put a conflict in a:
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo b >> a
Also add another file just in the merged in branch:
$ echo b >> b
$ hg ci -Am.
adding b
created new head
Go back and merge the anonymous head:
$ hg up 1
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ hg merge 2 --tool internal:local
1 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
Naturally at this point without the merge tool we'd get a conflict on a. But using the merge tool, we're telling Mercurial to take the version of the first parent on every file that the merged with cset has also touched.
$ hg st
M a
M b
$ cat a
a
a
$ cat b
b