chore(shell): add git find merge commit

This will output the git sha of the merge commit that a given commit is
part of. This can be used to get back to the pull request a commit was
part of. Or it can be used to help revert a hole changeset rather than
only one commit.

site-modules/core/files/bin/git-find-merge

@@ -0,0 +1,7 @@
+#!/bin/bash
+# See: https://stackoverflow.com/questions/8475448/find-merge-commit-which-includes-a-specific-commit
+
+commit=$1
+branch=${2:-HEAD}
+candidates=$(git rev-list $commit..$branch --ancestry-path | cat -n; git rev-list $commit..$branch --first-parent | cat -n)
+echo "$candidates" | sort -k2 -s | uniq -f1 -d | sort -n | tail -1 | cut -f2