travis-CI: mark rootdir as save1.26

fixes this error:
```
make[3]: Entering directory '/rootdir'

Creating ChangeLog

fatal: unsafe repository ('/rootdir' is owned by someone else)

To add an exception for this directory, call:

	git config --global --add safe.directory /rootdir

Failed to generate ChangeLog
```
A change was introduced in git 2.35.2 (and newer) to prevent a user from executing git commands in a repository owned by a different user.
See https://confluence.atlassian.com/bbkb/git-command-returns-fatal-error-about-the-repository-being-owned-by-someone-else-1167744132.html

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