Abort git commit if pre-commit applies formatting (#737)

In #660 we added a `.pre-commit` file that developers can use to
automatically apply formatting and update datestamps. It's a minor
annoyance that the commit still goes through even if changes were
applied. Then you either have separate "Update datestamps" /
"Formatting" commits or have to squash/fixup with `git rebase`.

This PR changes the pre-commit so that it aborts the commit if any
changes were made. Then you just re-run the `git commit` command with
the formatting/date changes included.

.pre-commit

@@ -2,6 +2,9 @@
 
 # An example pre-commit file for nextSIM-DG development
 
+BEFORE=$(mktemp)
+git diff > ${BEFORE}
+
 for FILE in $(git diff --cached --name-only | grep -iE '\.(cpp|cc|h|hpp)$'); do
   # Apply clang-format for linting
   clang-format -i ${FILE} --verbose
@@ -11,3 +14,12 @@ for FILE in $(git diff --cached --name-only | grep -iE '\.(cpp|cc|h|hpp)$'); do
   NEWLINE=$(printf "%s%$((SPACES))s%s\n" " @date" " " "$(date '+%d %b %Y')")
   sed -i~ "s/${LINE}/${NEWLINE}/" ${FILE}
 done
+
+# Abort commit if formatting was applied
+AFTER=$(mktemp)
+git diff > ${AFTER}
+if [ "$(diff nextsim_precommit_before.patch nextsim_precommit_after.patch)" ]; then
+  echo "Aborting git commit because the pre-commit hook made changes."
+  rm ${BEFORE} ${AFTER}
+  exit 1
+fi