Junio C Hamano <junkio@cox.net> writes:

quoted text
> The standard way to check if commit A is included in (i.e. is an
> ancestor of) commit B, without traversing the ancestry chain of
> B all the way down to the root commit, is to run:
>
> 	git merge-base --all A B
>
> and see if A appears in its output (if so, then A is an ancestor
> of B, otherwise it is not).  This is a pair-wise check, and for
> your purpose the check would become N*M operation (Yuck).
>
> The same check can be done in parallel with:
>
> 	git show-branch --independent A B C D...
>
> whose output would include A if the commit is not included in
> any of the other commits B C D...  This parallel traversal has a
> limit --- you can only check 25 branches at a time.

Well, I was silly.  If you want to see if A is an ancestor of
any of B C D..., the standard and most efficient way to do so is
with rev-list.

	git rev-list A --not B C D...

will show _nothing_ only when A is an ancestor of one (or more)
of B C D..., so you invoke it and upon getting the first line of
output you declare A cannot be removed without reading the
remainder of the output.

show-branch --independent is an overkill for your purpose as it
does not treat A any more special from others (iow, it checks if
B is contained in A C D..., C is contained in A B D... all in
parallel), and you are not interested in finding out how remote
refs are related with each other.

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