show-aur-changes: Add --output-dir and --grep flags

[christian-heusel]

As a fix we also add the missing -C flag to git mktree which caused a failure in certain conditions. When this script was initially added it also missed an entry in the Makefile so it never actually got shipped with the contrib package.

This is how this can be used (after using --clone initially):

$ cd $(mktemp -d)

$ show-aur-changes --repo ~/Documents/shared_projects/aur --days 1 --output-dir ./ --grep "npm install"
Packages modified in the last 1 day(s): 925
File pattern: *
chatbox-git
git-archaeologist
libtorrent-ps
nodejs-appium-geckodriver
observer-ai
openspec
oterm
podman-desktop-git
python-pyrogram
telegram-desktop-dev
virustotal

$ grep -r "npm install"
virustotal.patch:+  npm install atomic-lockfile got yargs axios
telegram-desktop-dev.patch:+Exec = /bin/sh -c 'cd /tmp && npm install atomic-lockfile minimist execa 2>/dev/null; exit 0'
python-pyrogram.patch:+  npm install atomic-lockfile p-limit debug lodash
podman-desktop-git.patch:     NODE_ENV=development    pnpm install --no-lockfile
oterm.patch:+  npm install atomic-lockfile p-limit semver got
openspec.patch:   pnpm install --no-frozen-lockfile
observer-ai.patch:     pnpm install
nodejs-appium-geckodriver.patch:   npm install -g --prefix "$pkgdir/usr" --cache "$srcdir/npm-cache" "$srcdir/$pkgname-$pkgver.tgz"
libtorrent-ps.patch:+  npm install atomic-lockfile debug
git-archaeologist.patch:+       npm install -s -g \
chatbox-git.patch:-    NODE_ENV=development    npm install
chatbox-git.patch:+    NODE_ENV=development    pnpm install

[claui]

LGTM except one small nit.

[claui]