Firstly, (the other) Adam is right that it doesn’t make sense to use --all for this: if you only want to see one branch like your question states, why ask for all branches?
Secondly, as already stated in comments to other answers, you don’t need --branches; just do git log mybranch.
Thirdly, I can explain why git log --branches=mybranch doesn’t work. The git-log(1) man page says:
--branches[=<pattern>]
Pretend as if all the refs in refs/heads are listed on
the command line as <commit>. If <pattern> is given,
limit branches to ones matching given shell glob. If
pattern lacks ?, *, or [, /* at the end is implied.
The last sentence is the crucial point here. If the <pattern> is just mybranch then there is no globbing character, so git-log interprets it as if you’d typed
git log --branches=mybranch/*
which only matches references under $repo/.git/refs/heads/mybranch/*, i.e. branches which begin with mybranch/.
There is a dirty hack to prevent the /* from being assumed:
git log --branches=[m]ybranch
but I can’t think of any good reason why you would want to do this rather than just typing
git log mybranch