Add Checking Commit Ancestry as a git til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_453 TILs and counting..._
+_454 TILs and counting..._
 
 ---
 
@@ -101,6 +101,7 @@ _453 TILs and counting..._
 - [Accessing a Lost Commit](git/accessing-a-lost-commit.md)
 - [Amend Author Of Previous Commit](git/amend-author-of-previous-commit.md)
 - [Caching Credentials](git/caching-credentials.md)
+- [Checking Commit Ancestry](git/checking-commit-ancestry.md)
 - [Checkout Old Version Of A File](git/checkout-old-version-of-a-file.md)
 - [Checkout Previous Branch](git/checkout-previous-branch.md)
 - [Clean Out All Local Branches](git/clean-out-all-local-branches.md)

git/checking-commit-ancestry.md

@@ -0,0 +1,27 @@
+# Checking Commit Ancestry
+
+I have two commit shas and I want to know if the first is an ancestor of the
+second. Put another way, is this first commit somewhere in the history of
+this other commit.
+
+Git's `merge-base` command combined with the `--is-ancestor` flag makes
+answering this question easy. Furthermore, because it is a plumbing command,
+it can be used in a script or sequence of commands as a switch based on the
+answer.
+
+Here is an example of this command in action:
+
+```bash
+$ git merge-base --is-ancestor head~ head && echo 'yes, it is'
+yes, it is
+$ git merge-base --is-ancestor head~ head~~ && echo 'yes, it is'
+```
+
+In the first command, `head~` is clearly an ancestor of `head`, so the
+`echo` command is triggered. In the second, `head~` is not an ancestor of
+`head~~` so the return status of 1 short-circuits the rest of the command.
+Hence, no `echo`.
+
+See `man git-merge-base` for more details.
+
+[source](http://stackoverflow.com/questions/18345157/how-can-i-tell-if-one-commit-is-an-ancestor-of-another-commit-or-vice-versa)