It is no longer in TortoiseSVN because Subversion 1.8 deprecated the --reintegrate option. Subversion 1.8 has automatic reintegration handling/detection.
See: Subversion 1.8 release notes, specifically the auto-reintegrate section
Here is the relevant area, for convenience:
Automatic reintegration merge (--reintegrate option deprecated)
During merges which merge all eligible revisions from another branch,
Subversion 1.8 will automatically decide whether or not the merge is
reintegrating a branch. Therefore, reintegrating a branch does no
longer require the --reintegrate option for correct operation.
The --reintegrate option of svn merge is now deprecated and its use is
discouraged. To reintegrate a branch, have a clean working copy of
trunk and run the following command in its top-level directory:
$ svn merge ^/branches/my-branch
This merge will still perform similar sanity checks which svn merge
--reintegrate performed in earlier releases:
- The working copy must not be a mixed-revision working copy.
- The working copy must not have switched subtrees.
- There must be no gaps in revision ranges merged from the reintegration target (e.g. the trunk) to the reintegration source
(i.e. the branch to be reintegrated).
If any of these conditions are detected, the merge is aborted and the
necessary steps must be taken to fix the problem before the branch can
be reintegrated. In contrast to a --reintegrate merge, an automatic
reintegration merge into a working copy with local modifications is
allowed.
Merging to-and-fro between two branches in any order is possible using
the automatic reintegration merge (the "keep-alive dance" is no longer
necessary). For best results, it is recommended to always merge all
eligible revisions, i.e. not using the -r or -c options of svn merge.
Merging just a subset of eligible revisions increases the likelihood
of problems during future merges.
Using --reintegrate in Subversion 1.8 will force a reintegration
merge, whether or not that's the right merge to perform in the given
situation.
In your case, you should do the following:
- Make sure you're using a clean, no modifications, up-to-date trunk working copy as you normally would
- TortoiseSVN -> Merge on this working copy root
- Select "Merge a range of revisions"
- Select the branch you are reintegrating
- Do not specify a revision range (to merge all eligible revisions)
- Subversion 1.8 should autodetect the reintegration and perform the same safety checks
- Proceed with your merge normally
According to the compatibility table, a Subversion 1.8 client can perform this auto-reintegrate as long as your Subversion server and repository format are each version 1.5 or later.
I haven't done an auto-reintegrate yet myself, I'm just going off the release notes.