Reduce CI build time (for md/txt/log)

Handle md/txt/log by appveyor in a special way.

Close #12592

appveyor.yml

@@ -35,10 +35,17 @@ before_build:
         Write-Output "Configuration : $env:configuration"
         Write-Output ""
         # XML validation mode
-        if ($env:APPVEYOR_PULL_REQUEST_NUMBER) {
+        $allowMaster = $true
+        if ($allowMaster -or $env:APPVEYOR_PULL_REQUEST_NUMBER) {
+            $files_nowork = "md|txt|log"
             $files_modified = @(git diff --name-only HEAD~1)
-            $files_notmached = @($files_modified | Where-Object {$_ -notmatch "\.xml$"})
-            if (($files_modified.length -gt 0 -and $files_notmached.length -eq 0) -or $env:APPVEYOR_REPO_COMMIT_MESSAGE -match "\[force xml\]") {
+            $files_notmached = @($files_modified | Where-Object {$_ -notmatch "\.(xml|$files_nowork)$"})
+            if (($files_modified.length -gt 0 -and $files_notmached.length -eq 0) -or $env:APPVEYOR_REPO_COMMIT_MESSAGE -match "\[force (xml|nowork)\]") {
+                if (@($files_modified | Where-Object {$_ -notmatch "\.($files_nowork)$"}).length -eq 0 -or $env:APPVEYOR_REPO_COMMIT_MESSAGE -match "\[force nowork\]") {
+                    Write-Output "Changed files on this commit don't require any additional tasks.`n"
+                    Exit-AppVeyorBuild
+                }
+                else {
                     Write-Output "XML validation mode`n"
                     if ("$env:platform/$env:configuration" -eq "Win32/Debug") {
                         if (@($files_modified | Where-Object {$_ -match "/functionList/"}).length -eq 0) {
@@ -60,8 +67,9 @@ before_build:
                         Exit-AppVeyorBuild
                     }
                 }
+            }
             elseif ($env:APPVEYOR_REPO_COMMIT_MESSAGE -match "\[xml\]" -and $env:APPVEYOR_REPO_COMMIT_MESSAGE -notmatch "\[force compile\]") {
-                throw "Changed files on this pull request require full build."
+                throw "Changed files on this commit require full build."
             }
         }