build.gradle: correctly determine git directory in case the repo is u…

…sed as a submodule

build.gradle

@@ -19,7 +19,22 @@ plugins {
 	id 'org.gradlex.extra-java-module-info' version '1.9'
 }
 
-def git = Grgit.open(dir: projectDir)
+def dotGitFile = new File("$projectDir/.git")
+def gitDir
+if (dotGitFile.isFile()) {
+	def gitDirConfigValue = dotGitFile.readLines().stream().mapMulti { line, downstream ->
+		def match = line =~ /^gitdir: (\S*)/
+		def value = match[0][1];
+		if (value) {
+			downstream.accept value
+		}
+	}.findFirst().orElseThrow({ new GradleException('gitdir not set') })
+	gitDir = "$projectDir/$gitDirConfigValue"
+} else {
+	gitDir = projectDir
+}
+
+def git = Grgit.open dir: gitDir
 version = "${git.describe(longDescr: true).replaceFirst('-', '.').replaceFirst('-g', '-')}${((!git.status().isClean()) ? '.dirty' : '')}"
 
 base {