Peter Wu [off-list ref] writes:
One note, the following command spits out master without complaining about the
non-existing branch name:
git branch --contains <id> master <non-existant branch name>
(the order of branches doesn't affect the result.)
That is perfectly normal.
What you gave after "--contains <id>" are *not* branch names. They
are patterns against branch names that fits the given criteria (in
this case "--contains <id>") are matched, and the branches that do
not match any of the patterns will not appear in the result.