Add githooks directory

Add a `githooks` directory. Copy the sample pre-commit hook into it.
Add a section to the README explaining how to take advantage of the
githooks.

The sample pre-commit hook includes checks for trailing whitespace that
we are seeing occasionally in PRs.

Done in preparation for adding a clippy githook.

README.md

@@ -125,6 +125,17 @@ In order to speed up the review process the CI pipeline can be run locally using
 skipped when using `act` due to caching being unsupported at this time. We do
 not *actively* support `act` but will merge PRs fixing `act` issues.
 
+### Githooks
+
+To assist devs in catching errors _before_ running CI we provide some githooks. If you do not
+already have locally configured githooks you can use the ones in this repository by running, in the
+root directory of the repository:
+```
+git config --local core.hooksPath githooks/
+```
+
+Alternatively add symlinks in your `.git/hooks` directory to any of the githooks we provide.
+
 ## Policy on Altcoins/Altchains
 
 Patches which add support for non-Bitcoin cryptocurrencies by adding constants

githooks/pre-commit

@@ -0,0 +1,46 @@
+#!/bin/sh
+#
+# Verify what is about to be committed. Called by "git commit" with no
+# arguments. The hook should exit with non-zero status after issuing an
+# appropriate message if it wants to stop the commit.
+
+if git rev-parse --verify HEAD >/dev/null 2>&1
+then
+	against=HEAD
+else
+	# Initial commit: diff against an empty tree object
+	against=$(git hash-object -t tree /dev/null)
+fi
+
+# If you want to allow non-ASCII filenames set this variable to true.
+allownonascii=$(git config --bool hooks.allownonascii)
+
+# Redirect output to stderr.
+exec 1>&2
+
+# Cross platform projects tend to avoid non-ASCII filenames; prevent
+# them from being added to the repository. We exploit the fact that the
+# printable range starts at the space character and ends with tilde.
+if [ "$allownonascii" != "true" ] &&
+	# Note that the use of brackets around a tr range is ok here, (it's
+	# even required, for portability to Solaris 10's /usr/bin/tr), since
+	# the square bracket bytes happen to fall in the designated range.
+	test $(git diff --cached --name-only --diff-filter=A -z $against |
+	  LC_ALL=C tr -d '[ -~]\0' | wc -c) != 0
+then
+	cat <<\EOF
+Error: Attempt to add a non-ASCII file name.
+
+This can cause problems if you want to work with people on other platforms.
+
+To be portable it is advisable to rename the file.
+
+If you know what you are doing you can disable this check using:
+
+  git config hooks.allownonascii true
+EOF
+	exit 1
+fi
+
+# If there are whitespace errors, print the offending file names and fail.
+exec git diff-index --check --cached $against --