[#215] Extracting info from git diff is not working as expected (#216)

FileInfoExtractor#getFilePathFromDiffPattern(String) checks
for the group pattern '+++ b/path/to/file'
within the git diff result in order to get the file path.

However, when a new and also empty file is added in between the
commit range to analysis, the file will appear in the git diff result,
but since there are no lines to display, the above group does not
get printed, thus 
FileInfoExtractor#getFilePathFromDiffPattern(String) is unable
to match the pattern, causing the program to crash.

Let's add a check to skip these new and empty files in 
the git diff result, since an empty file has no lines to 
attribute any contribution.

src/main/java/reposense/authorship/FileInfoExtractor.java

@@ -9,6 +9,7 @@
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
+import java.util.function.Predicate;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import java.util.regex.Matcher;
@@ -31,18 +32,23 @@ public class FileInfoExtractor {
 
     private static final String DIFF_FILE_CHUNK_SEPARATOR = "\ndiff --git a/.*\n";
     private static final String BINARY_FILE_SYMBOL = "\nBinary files ";
-    private static final String FILE_DELETED_METADATA = "deleted file mode 100644\n";
     private static final String LINE_CHUNKS_SEPARATOR = "\n@@ ";
     private static final String LINE_INSERTED_SYMBOL = "+";
     private static final String STARTING_LINE_NUMBER_GROUP_NAME = "startingLineNumber";
     private static final String FILE_CHANGED_GROUP_NAME = "filePath";
+    private static final String MATCH_GROUP_FAIL_MESSAGE_FORMAT = "Failed to match the %s group for:\n%s";
 
     private static final int LINE_CHANGED_HEADER_INDEX = 0;
 
     private static final Pattern STARTING_LINE_NUMBER_PATTERN = Pattern.compile(
             "-(\\d)+(,)?(\\d)* \\+(?<startingLineNumber>\\d+)(,)?(\\d)* @@");
     private static final Pattern FILE_CHANGED_PATTERN = Pattern.compile("\n(\\+){3} b/(?<filePath>.*)\n");
 
+    private static final Predicate<String> FILE_DELETED_PREDICATE = Pattern.compile(
+            "^deleted file mode [\\d]{6}\n").asPredicate();
+    private static final Predicate<String> NEW_EMPTY_FILE_PREDICATE = Pattern.compile(
+            "^new file mode [a-zA-Z0-9\n. ]*$").asPredicate();
+
     /**
      * Extracts a list of relevant files given in {@code config}.
      */
@@ -84,8 +90,9 @@ public static List<FileInfo> getEditedFileInfos(RepoConfiguration config, String
         String[] fileDiffResultList = fullDiffResult.split(DIFF_FILE_CHUNK_SEPARATOR);
 
         for (String fileDiffResult : fileDiffResultList) {
-            // file deleted or is a binary file, skip it
-            if (fileDiffResult.startsWith(FILE_DELETED_METADATA) || fileDiffResult.contains(BINARY_FILE_SYMBOL)) {
+            // file deleted, is binary file or is new and empty file, skip it
+            if (FILE_DELETED_PREDICATE.test(fileDiffResult) || fileDiffResult.contains(BINARY_FILE_SYMBOL)
+                    || NEW_EMPTY_FILE_PREDICATE.test(fileDiffResult)) {
                 continue;
             }
 
@@ -202,6 +209,7 @@ private static String getFilePathFromDiffPattern(String fileDiffResult) {
         Matcher filePathMatcher = FILE_CHANGED_PATTERN.matcher(fileDiffResult);
 
         if (!filePathMatcher.find()) {
+            logger.severe(String.format(MATCH_GROUP_FAIL_MESSAGE_FORMAT, "file path", fileDiffResult));
             throw new AssertionError("Should not have error matching file path pattern inside file diff result!");
         }
 
@@ -216,6 +224,7 @@ private static int getStartingLineNumber(String linesChanged) {
         Matcher chunkHeaderMatcher = STARTING_LINE_NUMBER_PATTERN.matcher(linesChanged);
 
         if (!chunkHeaderMatcher.find()) {
+            logger.severe(String.format(MATCH_GROUP_FAIL_MESSAGE_FORMAT, "line changed", linesChanged));
             throw new AssertionError("Should not have error matching line number pattern inside chunk header!");
         }