+A common idiom to check "fast-forward-ness" between two commits A
+and B is (or at least used to be) to compute the merge base between
+A and B, and check if it is the same as A, in which case, A is an
+ancestor of B. You will see this idiom used often in older scripts.
+
+ A=$(git rev-parse --verify A)
+ if test "$A" = "$(git merge-base A B)"
+ then
+ ... A is an ancestor of B ...
+ fi
+
+In modern git, you can say this in a more direct way:
+
+ if git merge-base --is-ancestor A B
+ then
+ ... A is an ancestor of B ...
+ fi
+
+instead.
+
+