Add a script to help find screenshot changes.

This script is a minor modification of a script I wrote in October 2017,
which was part of the internal Git repository prior to open sourcing.
I'm not preserving its commit history because that script doesn't have
any history of interest -- just adding comments and examples over time.

Author: amartini51

bin/find_screenshot_changes

@@ -0,0 +1,41 @@
+#! /usr/bin/env zsh
+
+setopt ERR_EXIT NO_UNSET PIPE_FAIL
+
+# This script helps identify changes that show up in screenshots.
+#
+# Set $COMMIT to the oldest commit this script should consider:
+# ideally, that's the commit when the current screenshots were taken.
+#
+# Lines in the output that start with whitespace are unchanged since $COMMIT.
+# Lines that start with a commit ID were changed in that commit, after $COMMIT.
+#
+# To check whether we need to re-shoot the screenshots,
+# run this script and check whether any lines start with commit IDs.
+# For example:
+#
+#     ./bin/find_screenshot_changes.zsh | grep -v '^ '
+#
+# To show those commits that made scheenshot changes:
+#
+#     ./bin/find_screenshot_changes.zsh |
+#     cut -d ' ' -f 1 |
+#     sort -u |
+#     xargs -L1 git --no-pager log -1
+
+BEGIN='<!-- Apple Books screenshot begins here -->'
+END='<!-- Apple Books screenshot ends here -->'
+
+# The commit when these comment markers were originally added.
+# COMMIT=64627a260ed600015dbad1a853d8b85f611b61d4
+
+# The commit when the content was converted from RST to markdown.
+COMMIT=96f0925407c6bd9eadd9d58d253bad3e1ef7a9f2
+
+cd Sources/TSPL/TSPL.docc
+git grep --files-with-matches $BEGIN | while IFS= read -r FILE
+do
+    git --no-pager blame -bs $COMMIT.. -L /$BEGIN/,/$END/ $FILE
+    echo
+    echo
+done