hook: provide GIT_HOOK for all hooks

In order to allow users to use one executable for multiple hooks,
provide a GIT_HOOK variable that is set to the hook event that triggered
it.

Signed-off-by: John Cai <johncai86@gmail.com>

Documentation/githooks.txt

@@ -31,6 +31,10 @@ Hooks can get their arguments via the environment, command-line
 arguments, and stdin. See the documentation for each hook below for
 details.
 
+The `$GIT_HOOK` environment variable is passed to all hooks and holds the
+triggering hook event, eg: `pre-commit`, `update`, etc. This allows one
+executable to be used for multiple hooks.
+
 `git init` may copy hooks to the new repository, depending on its
 configuration. See the "TEMPLATE DIRECTORY" section in
 linkgit:git-init[1] for details. When the rest of this document refers

hook.c

@@ -144,6 +144,8 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
 		cb_data.hook_path = abs_path.buf;
 	}
 
+	strvec_pushf(&cb_data.options->env,"GIT_HOOK=%s", hook_name);
+
 	run_processes_parallel_tr2(jobs,
 				   pick_next_hook,
 				   notify_start_failure,

t/t1800-hook.sh

@@ -38,6 +38,18 @@ test_expect_success 'git hook run: basic' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git hook run: $GIT_HOOK' '
+	test_hook test-hook <<-EOF &&
+	printenv GIT_HOOK
+	EOF
+
+	cat >expect <<-\EOF &&
+	test-hook
+	EOF
+	git hook run test-hook 2>actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git hook run: stdout and stderr both write to our stderr' '
 	test_hook test-hook <<-EOF &&
 	echo >&1 Will end up on stderr