Merge 7a4f812a5f into 9ed9b0b4e1

Update the README to prevent partial upgrades.

Partial upgrades are dangerous on Arch Linux.
Partial upgrades may occur when a `-y` operation is given without a corresponding `-u` operation.

Quote from the ArchWiki:
> Do not use:
> `pacman -Sy package`
> `pacman -Sy` followed by `pacman -S` package (Note the absence of `-S**u**` in the installation of the package.)
> `pacman -Syuw` (Note that `pacman -Syuw` does imply the same risks like pacman -Sy`, as it will update the pacman sync database without installing the newer packages.)

https://wiki.archlinux.org/title/System_maintenance#Partial_upgrades_are_unsupported

README.md

@@ -35,10 +35,10 @@ If you are migrating from another AUR helper, you can simply install Yay with th
 The initial installation of Yay can be done by cloning the PKGBUILD and
 building with makepkg:
 
-We start with updating the package lists and make sure we have the `base-devel` package group installed.
+We make sure we have the `base-devel` package group installed.
 
 ```sh
-pacman -Sy --needed git base-devel
+pacman -S --needed git base-devel
 git clone https://aur.archlinux.org/yay.git
 cd yay
 makepkg -si

local_install_test.go

@@ -73,6 +73,7 @@ func TestIntegrationLocalInstall(t *testing.T) {
 
 	wantCapture := []string{
 		"makepkg --packagelist",
+		"git -C testdata/jfin branch --show-current",
 		"git -C testdata/jfin git reset --hard HEAD",
 		"git -C testdata/jfin git merge --no-edit --ff",
 		"makepkg --packagelist",
@@ -337,6 +338,7 @@ func TestIntegrationLocalInstallNeeded(t *testing.T) {
 
 	wantCapture := []string{
 		"makepkg --packagelist",
+		"git -C testdata/jfin branch --show-current",
 		"git -C testdata/jfin git reset --hard HEAD",
 		"git -C testdata/jfin git merge --no-edit --ff",
 		"makepkg --packagelist",
@@ -510,6 +512,7 @@ func TestIntegrationLocalInstallGenerateSRCINFO(t *testing.T) {
 	wantCapture := []string{
 		"makepkg --printsrcinfo",
 		"makepkg --packagelist",
+		"git -C testdata/jfin branch --show-current",
 		"git -C testdata/jfin git reset --hard HEAD",
 		"git -C testdata/jfin git merge --no-edit --ff",
 		"makepkg --packagelist",
@@ -795,6 +798,7 @@ func TestIntegrationLocalInstallWithDepsProvides(t *testing.T) {
 	}
 
 	wantCapture := []string{
+		"git -C testdata/cephbin branch --show-current",
 		"git -C testdata/cephbin git reset --hard HEAD",
 		"git -C testdata/cephbin git merge --no-edit --ff",
 		"makepkg --packagelist",
@@ -923,8 +927,10 @@ func TestIntegrationLocalInstallTwoSrcInfosWithDeps(t *testing.T) {
 	}
 
 	wantCapture := []string{
+		"git -C testdata/gourou branch --show-current",
 		"git -C testdata/gourou git reset --hard HEAD",
 		"git -C testdata/gourou git merge --no-edit --ff",
+		"git -C testdata/libzip-git branch --show-current",
 		"git -C testdata/libzip-git git reset --hard HEAD",
 		"git -C testdata/libzip-git git merge --no-edit --ff",
 		"makepkg --packagelist",

pkg/sync/workdir/merge.go

@@ -27,9 +27,27 @@ func gitMerge(ctx context.Context, cmdBuilder exe.ICmdBuilder, dir string) error
 	return nil
 }
 
+func pkgbuildCanMerge(ctx context.Context, cmdBuilder exe.ICmdBuilder, dir string) (bool, error) {
+	stdout, stderr, err := cmdBuilder.Capture(
+		cmdBuilder.BuildGitCmd(ctx,
+			dir, "branch", "--show-current"))
+	if err != nil {
+		return false, errors.New(gotext.Get("error showing branch %s: %s", dir, stderr))
+	}
+
+	return stdout != "", nil
+}
+
 func mergePkgbuilds(ctx context.Context, cmdBuilder exe.ICmdBuilder, pkgbuildDirs map[string]string) error {
 	for _, dir := range pkgbuildDirs {
-		err := gitMerge(ctx, cmdBuilder, dir)
+		canMerge, err := pkgbuildCanMerge(ctx, cmdBuilder, dir)
+		if err != nil {
+			return err
+		}
+		if !canMerge {
+			continue
+		}
+		err = gitMerge(ctx, cmdBuilder, dir)
 		if err != nil {
 			return err
 		}

sync_test.go

@@ -545,6 +545,7 @@ pkgname = python-vosk
 	require.NoError(t, err)
 
 	wantCapture := []string{
+		"/usr/bin/git -C testdata/vosk-api branch --show-current",
 		"/usr/bin/git -C /testdir/vosk-api reset --hard HEAD",
 		"/usr/bin/git -C /testdir/vosk-api merge --no-edit --ff",
 		"makepkg --packagelist", "makepkg --packagelist",