What if one branch has two heads? I came to this situation weeks ago for some reason I didn't merged them back then and just continued developing on one head. Now I want to get rid of the other head. What should I do? Should I just merge them after so many changesets?
So you have this:
o--o--o--A--B <- older unnecessary head
\
1--2--3--4--5--6 <- newer ‘good’ head
...and you don't need A and B, absolutely, 100% sure. If you aren't sure, and there are possibly salvageable things in A and B, better merge the heads to combine the changes. As Aaron said, Mercurial is good at it.
Now you have two options:
B.If changesets A and B are present in other repositories you don't control, e.g. if other people pulled A and B into their repositories, or you pushed A and B to a public repository (say, Bitbucket), then you've released A and B into the wild, and can't get rid of them. You should do a dummy merge:
$ hg up 6
$ hg --config ui.merge=internal:local merge
This will ignore any changes from A and B when merging.
If, on the other hand, A and B are private, you can either strip them:
$ hg strip A
(Rev A and descendants stripped; enable the MQ extension to make strip available.)
Or, make a new clone of your repository without changesets A and B:
$ hg clone myrepo myrepo2-clone -r 6
(Only rev 6 and ancestors added to the clone.)