restore: default to HEAD when combining --staged and --worktree

By default, files are restored from the index for --worktree, and from
HEAD for --staged. When --worktree and --staged are combined, --source
must be specified to disambiguate the restore source[1], thus making it
cumbersome to restore a file in both the worktree and the index.

However, HEAD is also a reasonable default for --worktree when combined
with --staged, so make it the default anytime --staged is used (whether
combined with --worktree or not).

[1]: Due to an oversight, the --source requirement, though documented,
is not actually enforced.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-restore.txt

@@ -22,9 +22,8 @@ The command can also be used to restore the content in the index with
 `--staged`, or restore both the working tree and the index with
 `--staged --worktree`.
 
-By default, the restore sources for working tree and the index are the
-index and `HEAD` respectively. `--source` could be used to specify a
-commit as the restore source.
+By default, if `--staged` is given, the contents are restored from `HEAD`,
+otherwise from the index. Use `--source` to restore from a different commit.
 
 See "Reset, restore and revert" in linkgit:git[1] for the differences
 between the three commands.
@@ -39,10 +38,8 @@ OPTIONS
 	tree. It is common to specify the source tree by naming a
 	commit, branch or tag associated with it.
 +
-If not specified, the default restore source for the working tree is
-the index, and the default restore source for the index is
-`HEAD`. When both `--staged` and `--worktree` are specified,
-`--source` must also be specified.
+If not specified, the contents are restored from `HEAD` if `--staged` is
+given, otherwise from the index.
 
 -p::
 --patch::

builtin/checkout.c

@@ -1581,10 +1581,10 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 	if (opts->checkout_index < 0 || opts->checkout_worktree < 0)
 		BUG("these flags should be non-negative by now");
 	/*
-	 * convenient shortcut: "git restore --staged" equals
-	 * "git restore --staged --source HEAD"
+	 * convenient shortcut: "git restore --staged [--worktree]" equals
+	 * "git restore --staged [--worktree] --source HEAD"
 	 */
-	if (!opts->from_treeish && opts->checkout_index && !opts->checkout_worktree)
+	if (!opts->from_treeish && opts->checkout_index)
 		opts->from_treeish = "HEAD";
 
 	/*

t/t2070-restore.sh

@@ -69,6 +69,17 @@ test_expect_success 'restore --staged uses HEAD as source' '
 	test_cmp expected actual
 '
 
+test_expect_success 'restore --worktree --staged uses HEAD as source' '
+	test_when_finished git reset --hard &&
+	git show HEAD:./first.t >expected &&
+	echo dirty >>first.t &&
+	git add first.t &&
+	git restore --worktree --staged first.t &&
+	git show :./first.t >actual &&
+	test_cmp expected actual &&
+	test_cmp expected first.t
+'
+
 test_expect_success 'restore --ignore-unmerged ignores unmerged entries' '
 	git init unmerged &&
 	(