Refine incremental logs

1. mkdir() before writing logs. In some cases, `build` isn't created by
   Gradle.
2. Add a timestamp for each build.

Author: ting-yuan

compiler-plugin/src/main/kotlin/com/google/devtools/ksp/Incremental.kt

@@ -166,6 +166,9 @@ class IncrementalContext(
 
     private val baseDir = options.projectBaseDir
 
+    private val logsDir = File(baseDir, "build").apply { mkdir() }
+    private val buildTime = Date().time
+
     private val modified = options.knownModified.map{ it.relativeTo(baseDir) }.toSet()
     private val removed = options.knownRemoved.map { it.relativeTo(baseDir) }.toSet()
 
@@ -262,7 +265,8 @@ class IncrementalContext(
         if (!options.incrementalLog)
             return
 
-        val logFile = File(options.projectBaseDir, "build/kspDirtySetByDeps.log")
+        val logFile = File(logsDir, "kspDirtySetByDeps.log")
+        logFile.appendText("=== Build $buildTime ===\n")
         logFile.appendText("All Files\n")
         ksFiles.forEach { logFile.appendText("  ${it.relativeFile}\n") }
         logFile.appendText("Modified\n")
@@ -289,7 +293,8 @@ class IncrementalContext(
         }
         val outputsToRemove = allOutputs - validOutputs
 
-        val logFile = File(options.projectBaseDir, "build/kspDirtySetByOutputs.log")
+        val logFile = File(logsDir, "kspDirtySetByOutputs.log")
+        logFile.appendText("=== Build $buildTime ===\n")
         logFile.appendText("Dirty sources\n")
         dirtyFiles.forEach { logFile.appendText("  $it\n") }
         logFile.appendText("Outputs to remove\n")
@@ -301,7 +306,8 @@ class IncrementalContext(
         if (!options.incrementalLog)
             return
 
-        val logFile = File(options.projectBaseDir, "build/kspSourceToOutputs.log")
+        val logFile = File(logsDir, "kspSourceToOutputs.log")
+        logFile.appendText("=== Build $buildTime ===\n")
         logFile.appendText("All outputs\n")
         sourceToOutputsMap.keys.forEach { source ->
             logFile.appendText("  $source:\n")
@@ -316,7 +322,8 @@ class IncrementalContext(
         if (!options.incrementalLog)
             return
 
-        val logFile = File(options.projectBaseDir, "build/kspDirtySet.log")
+        val logFile = File(logsDir, "kspDirtySet.log")
+        logFile.appendText("=== Build $buildTime ===\n")
         logFile.appendText("Dirty:\n")
         files.forEach {
             logFile.appendText("  ${it.relativeFile}\n")