Don't scan hidden directories in arduino-builder (.git, etc.) (#366)

* Don't scan hidden directories in arduino-builder

Arduino-builder scans the core to determine if core config files have
changed between invocations.  Unfortunately, it goes into SCCS dirs
such as .git (which can have 100,000+ files).  This is wasted effort
and can cause massive amounts of runtime and memory use when core
developers are building in their git clones.

Use a new function already added by the builder/utils.go to determine
if a file or folder is SCCS, and if so skip it in the rebuild-required
check.

Fixes arduino/arduino-builder#327

Signed-off-by: Earle F. Philhower, III <earlephilhower@yahoo.com>

* Add comment to force rebuild

legacy/builder/builder_utils/utils.go

@@ -154,11 +154,14 @@ func findAllFilesInFolder(sourcePath string, recurse bool) ([]string, error) {
 		}
 
 		for _, folder := range folders {
-			otherSources, err := findAllFilesInFolder(filepath.Join(sourcePath, folder.Name()), recurse)
-			if err != nil {
-				return nil, i18n.WrapError(err)
+			if !utils.IsSCCSOrHiddenFile(folder) {
+				// Skip SCCS directories as they do not influence the build and can be very large
+				otherSources, err := findAllFilesInFolder(filepath.Join(sourcePath, folder.Name()), recurse)
+				if err != nil {
+					return nil, i18n.WrapError(err)
+				}
+				sources = append(sources, otherSources...)
 			}
-			sources = append(sources, otherSources...)
 		}
 	}