git-am: teach git-am to apply a patch to an unborn branch

People sometimes wonder why they cannot apply a patch that only
creates new files to an unborn branch.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -36,6 +36,13 @@ cd_to_toplevel
 git var GIT_COMMITTER_IDENT >/dev/null ||
 	die "You need to set your committer info first"
 
+if git rev-parse --verify -q HEAD >/dev/null
+then
+	HAS_HEAD=yes
+else
+	HAS_HEAD=
+fi
+
 sq () {
 	for sqarg
 	do
@@ -290,16 +297,26 @@ else
 		: >"$dotest/rebasing"
 	else
 		: >"$dotest/applying"
-		git update-ref ORIG_HEAD HEAD
+		if test -n "$HAS_HEAD"
+		then
+			git update-ref ORIG_HEAD HEAD
+		else
+			git update-ref -d ORIG_HEAD >/dev/null 2>&1
+		fi
 	fi
 fi
 
 case "$resolved" in
 '')
-	files=$(git diff-index --cached --name-only HEAD --) || exit
+	case "$HAS_HEAD" in
+	'')
+		files=$(git ls-files) ;;
+	?*)
+		files=$(git diff-index --cached --name-only HEAD --) ;;
+	esac || exit
 	if test "$files"
 	then
-		: >"$dotest/dirtyindex"
+		test -n "$HAS_HEAD" && : >"$dotest/dirtyindex"
 		die "Dirty index: cannot apply patches (dirty: $files)"
 	fi
 esac
@@ -541,18 +558,20 @@ do
 	fi
 
 	tree=$(git write-tree) &&
-	parent=$(git rev-parse --verify HEAD) &&
 	commit=$(
 		if test -n "$ignore_date"
 		then
 			GIT_AUTHOR_DATE=
 		fi
+		parent=$(git rev-parse --verify -q HEAD) ||
+		echo >&2 "applying to an empty history"
+
 		if test -n "$committer_date_is_author_date"
 		then
 			GIT_COMMITTER_DATE="$GIT_AUTHOR_DATE"
 			export GIT_COMMITTER_DATE
 		fi &&
-		git commit-tree $tree -p $parent <"$dotest/final-commit"
+		git commit-tree $tree ${parent:+-p $parent} <"$dotest/final-commit"
 	) &&
 	git update-ref -m "$GIT_REFLOG_ACTION: $FIRSTLINE" HEAD $commit $parent ||
 	stop_here $this

t/t4150-am.sh

@@ -290,4 +290,19 @@ test_expect_success 'am --ignore-date' '
 	echo "$at" | grep "+0000"
 '
 
+test_expect_success 'am into an unborn branch' '
+	rm -fr subdir &&
+	mkdir -p subdir &&
+	git format-patch --numbered-files -o subdir -1 first &&
+	(
+		cd subdir &&
+		git init &&
+		git am 1
+	) &&
+	result=$(
+		cd subdir && git rev-parse HEAD^{tree}
+	) &&
+	test "z$result" = "z$(git rev-parse first^{tree})"
+'
+
 test_done