tools: add an utility cmd/tools/git_pre_commit_hook.vsh script

That script can be used to ensure that all commited V files are vfmt-ed,
i.e. it will run `v fmt -w ` on them before commiting them.

To use the script in your V project, you need to be in the main folder
of your project, then do the equivalent of:
```sh
cp /PATH/TO_YOUR/V/cmd/tools/git_pre_commit_hook.vsh .git/hooks/pre-commit
chmod 755 .git/hooks/pre-commit
```

Note: you can use this command:
`git config --bool --add hooks.stopCommitOfNonVfmtedVFiles true`
... to make it just *prevent* the commiting of unformatted .v files,
i.e. stop the commiting, if they are not, but *without modifying them*
automatically (you will then need to run `v fmt -w` on them manually).

Note 2: Git supports the option `--no-verify`, to temporarily disable all hooks.

cmd/tools/git_pre_commit_hook.vsh

@@ -0,0 +1,49 @@
+#!/usr/bin/env -S v -raw-vsh-tmp-prefix tmp
+
+import os
+import term
+
+// This script can be used to ensure that all commited V files are vfmt-ed automatically.
+// By default, once setup, it will run `v fmt -w` on them, before commiting them.
+
+// To use the script in your V project, you need to be in the main folder
+// of your project, then do the equivalent of:
+// ```sh
+// cp /PATH/TO_YOUR/V/cmd/tools/git_pre_commit_hook.vsh .git/hooks/pre-commit
+// chmod 755 .git/hooks/pre-commit
+// ```
+//
+// Note: you can use this command:
+// `git config --bool --add hooks.stopCommitOfNonVfmtedVFiles true`
+// ... to make it just *prevent* the commiting of unformatted .v files,
+// i.e. stop the commiting, if they are not, but *without modifying them*
+// automatically (you will then need to run `v fmt -w` on them manually).
+//
+// Note 2: Git supports skipping the hooks, by passing the `--no-verify` option.
+// That can be used to commit some .v files that are not formatted, without removing
+// the hook.
+
+fn main() {
+	// This hook cares only about the changed V files, that will be commited, as reported by git itself:
+	changed := os.execute("git diff --cached --name-only --diff-filter=ACMR -- '*.v' '*.vsh' '*.vv'")
+	if changed.output == '' {
+		eprintln('>>> 0 changed V files found.')
+		exit(0)
+	}
+	vfiles := changed.output.trim_space().split('\n')
+	configured_stop_commiting := os.execute('git config --bool hooks.stopCommitOfNonVfmtedVFiles')
+	if configured_stop_commiting.output.trim_space().bool() {
+		verify_result := os.execute('v fmt -verify ${vfiles.join(' ')}')
+		if verify_result.exit_code != 0 {
+			eprintln(verify_result.output)
+		}
+		exit(verify_result.exit_code)
+	} else {
+		eprintln('The V pre commit hook will format $vfiles.len V file(s):')
+		for vfile in vfiles {
+			eprintln('    ${term.bold('$vfile')}')
+		}
+		os.system('v fmt -w ${vfiles.join(' ')}')
+		os.system('git add ${vfiles.join(' ')}')
+	}
+}