Refactor git version functions and check compatibility (#29155)

Introduce a new function checkGitVersionCompatibility, when the git
version can't be used by Gitea, tell the end users to downgrade or
upgrade. The refactored functions are related to make the code easier to
test.

And simplify the comments for "safe.directory"

---------

Co-authored-by: delvh <dev.lh@web.de>

modules/git/git_test.go

@@ -13,6 +13,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 
+	"github.com/hashicorp/go-version"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -93,3 +94,25 @@ func TestSyncConfig(t *testing.T) {
 	assert.True(t, gitConfigContains("[sync-test]"))
 	assert.True(t, gitConfigContains("cfg-key-a = CfgValA"))
 }
+
+func TestParseGitVersion(t *testing.T) {
+	v, err := parseGitVersionLine("git version 2.29.3")
+	assert.NoError(t, err)
+	assert.Equal(t, "2.29.3", v.String())
+
+	v, err = parseGitVersionLine("git version 2.29.3.windows.1")
+	assert.NoError(t, err)
+	assert.Equal(t, "2.29.3", v.String())
+
+	_, err = parseGitVersionLine("git version")
+	assert.Error(t, err)
+
+	_, err = parseGitVersionLine("git version windows")
+	assert.Error(t, err)
+}
+
+func TestCheckGitVersionCompatibility(t *testing.T) {
+	assert.NoError(t, checkGitVersionCompatibility(version.Must(version.NewVersion("2.43.0"))))
+	assert.ErrorContains(t, checkGitVersionCompatibility(version.Must(version.NewVersion("2.43.1"))), "regression bug of GIT_FLUSH")
+	assert.NoError(t, checkGitVersionCompatibility(version.Must(version.NewVersion("2.43.2"))))
+}