Merge branch 'jc/merge-bases-paint-fix'

diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt
index b295bf83302de3fdac2833df756799a7aad6b404..87842e33f8aebc7d27fa732af1de15f0e4175c07 100644 (file)
--- a/Documentation/git-merge-base.txt
+++ b/Documentation/git-merge-base.txt
@@ -11,6 +11,7 @@ SYNOPSIS
 [verse]
 'git merge-base' [-a|--all] <commit> <commit>...
 'git merge-base' [-a|--all] --octopus <commit>...
+'git merge-base' --is-ancestor <commit> <commit>
 'git merge-base' --independent <commit>...
 
 DESCRIPTION
@@ -50,6 +51,12 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option.
        from any other.  This mimics the behavior of 'git show-branch
        --independent'.
 
+--is-ancestor::
+       Check if the first <commit> is an ancestor of the second <commit>,
+       and exit with status 0 if true, or with status 1 if not.
+       Errors are signaled by a non-zero status that is not 1.
+
+
 OPTIONS
 -------
 -a::
@@ -110,6 +117,27 @@ both '1' and '2' are merge-bases of A and B.  Neither one is better than
 the other (both are 'best' merge bases).  When the `--all` option is not given,
 it is unspecified which best one is output.
 
+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.
+
+
 See also
 --------
 linkgit:git-rev-list[1],