Add pre-commit hook

This is something we can use to check formatting before
any commits are pushed.

build-aux/hooks/pre-commit.hook

@@ -0,0 +1,57 @@
+#!/bin/sh
+# Source: https://gitlab.gnome.org/GNOME/fractal/blob/master/hooks/pre-commit.hook
+
+install_rustfmt() {
+    if ! which rustup &> /dev/null; then
+        curl https://sh.rustup.rs -sSf  | sh -s -- -y
+        export PATH=$PATH:$HOME/.cargo/bin
+        if ! which rustup &> /dev/null; then
+            echo "Failed to install rustup. Performing the commit without style checking."
+            exit 0
+        fi
+    fi
+
+    if ! rustup component list|grep rustfmt &> /dev/null; then
+        echo "Installing rustfmt…"
+        rustup component add rustfmt
+    fi
+}
+
+if ! which cargo >/dev/null 2>&1 || ! cargo fmt --help >/dev/null 2>&1; then
+    echo "Unable to check Fractal’s code style, because rustfmt could not be run."
+
+    if [ ! -t 1 ]; then
+        # No input is possible
+        echo "Performing commit."
+        exit 0
+    fi
+
+    echo ""
+    echo "y: Install rustfmt via rustup"
+    echo "n: Don't install rustfmt and perform the commit"
+    echo "Q: Don't install rustfmt and abort the commit"
+
+    echo ""
+    while true
+    do
+        echo -n "Install rustfmt via rustup? [y/n/Q]: "; read yn < /dev/tty
+        case $yn in
+            [Yy]* ) install_rustfmt; break;;
+            [Nn]* ) echo "Performing commit."; exit 0;;
+            [Qq]* | "" ) echo "Aborting commit."; exit -1 >/dev/null 2>&1;;
+            * ) echo "Invalid input";;
+        esac
+    done
+
+fi
+
+echo "--Checking style--"
+cargo fmt --all -- --check
+if test $? != 0; then
+    echo "--Checking style fail--"
+    echo "Please fix the above issues, either manually or by running: cargo fmt --all"
+
+    exit -1
+else
+    echo "--Checking style pass--"
+fi