Simplify permission preservation to follow Git's approach

Following the reviewer's suggestion, this commit simplifies Unix permission
handling to only preserve the owner executable bit, matching Git's approach.

Git stores file permissions as either 100644 (non-executable) or 100755
(executable), focusing on the functional aspect (executability) while
ignoring platform-specific permission details that are generally irrelevant
for cross-platform builds.

Changes:
- permissionsToMode(): Only checks OWNER_EXECUTE permission, returns
  0100755 if executable, 0100644 otherwise
- modeToPermissions(): Restores either rwxr-xr-x (0755) or rw-r--r-- (0644)
  based on owner executable bit

Benefits:
- Simpler, more portable implementation
- Matches proven Git model for cross-platform permission handling
- Solves issue #214 (executable shell scripts) without over-engineering
- Avoids cache invalidation from irrelevant permission differences

All tests pass, including CacheUtilsPermissionsTest which verifies:
- Permission changes affect cache hash when preservePermissions=true
- Permissions are not preserved when preservePermissions=false

Author: cowwoc

src/main/java/org/apache/maven/buildcache/CacheUtils.java

@@ -255,80 +255,61 @@ public static <T> void debugPrintCollection(
     }
 
     /**
-     * Convert POSIX file permissions to Unix mode integer.
+     * Convert POSIX file permissions to Unix mode integer, following Git's approach of only
+     * preserving the owner executable bit.
+     *
+     * <p>Git stores file permissions as either {@code 100644} (non-executable) or {@code 100755}
+     * (executable). This simplified approach focuses on the functional aspect (executability)
+     * while ignoring platform-specific permission details that are generally irrelevant for
+     * cross-platform builds.</p>
      *
      * @param permissions POSIX file permissions
-     * @return Unix mode as integer (e.g., {@code 0100755} for regular file with {@code rwxr-xr-x})
+     * @return Unix mode: {@code 0100755} if owner-executable, {@code 0100644} otherwise
      */
     private static int permissionsToMode(Set<PosixFilePermission> permissions) {
-        // Start with regular file type (0100000 in octal)
-        int mode = 0100000;
-
-        if (permissions.contains(PosixFilePermission.OWNER_READ)) {
-            mode |= 0400;
-        }
-        if (permissions.contains(PosixFilePermission.OWNER_WRITE)) {
-            mode |= 0200;
-        }
+        // Following Git's approach: preserve only the owner executable bit
+        // Git uses 100644 (rw-r--r--) for regular files and 100755 (rwxr-xr-x) for executables
         if (permissions.contains(PosixFilePermission.OWNER_EXECUTE)) {
-            mode |= 0100;
-        }
-        if (permissions.contains(PosixFilePermission.GROUP_READ)) {
-            mode |= 0040;
-        }
-        if (permissions.contains(PosixFilePermission.GROUP_WRITE)) {
-            mode |= 0020;
-        }
-        if (permissions.contains(PosixFilePermission.GROUP_EXECUTE)) {
-            mode |= 0010;
+            return 0100755; // Regular file, executable
+        } else {
+            return 0100644; // Regular file, non-executable
         }
-        if (permissions.contains(PosixFilePermission.OTHERS_READ)) {
-            mode |= 0004;
-        }
-        if (permissions.contains(PosixFilePermission.OTHERS_WRITE)) {
-            mode |= 0002;
-        }
-        if (permissions.contains(PosixFilePermission.OTHERS_EXECUTE)) {
-            mode |= 0001;
-        }
-        return mode;
     }
 
     /**
-     * Convert Unix mode integer to POSIX file permissions.
+     * Convert Unix mode integer to POSIX file permissions, following Git's simplified approach.
+     *
+     * <p>This method interprets the two Git-standard modes:</p>
+     * <ul>
+     *   <li>{@code 0100755} - Executable file: sets owner+group+others read/execute, owner write</li>
+     *   <li>{@code 0100644} - Regular file: sets owner+group+others read, owner write</li>
+     * </ul>
      *
-     * @param mode Unix mode (e.g., {@code 0100755} for regular file with {@code rwxr-xr-x})
+     * <p>The key distinction is the presence of the execute bit. Other permission variations
+     * are normalized to these two standard patterns for portability.</p>
+     *
+     * @param mode Unix mode (should be either {@code 0100755} or {@code 0100644})
      * @return Set of POSIX file permissions
      */
     private static Set<PosixFilePermission> modeToPermissions(int mode) {
         Set<PosixFilePermission> permissions = new HashSet<>();
-        // Extract permission bits (lower 9 bits), ignoring file type bits
-        if ((mode & 0400) != 0) {
+
+        // Check owner executable bit (following Git's approach)
+        if ((mode & 0100) != 0) {
+            // Mode 100755: rwxr-xr-x (executable file)
             permissions.add(PosixFilePermission.OWNER_READ);
-        }
-        if ((mode & 0200) != 0) {
             permissions.add(PosixFilePermission.OWNER_WRITE);
-        }
-        if ((mode & 0100) != 0) {
             permissions.add(PosixFilePermission.OWNER_EXECUTE);
-        }
-        if ((mode & 0040) != 0) {
             permissions.add(PosixFilePermission.GROUP_READ);
-        }
-        if ((mode & 0020) != 0) {
-            permissions.add(PosixFilePermission.GROUP_WRITE);
-        }
-        if ((mode & 0010) != 0) {
             permissions.add(PosixFilePermission.GROUP_EXECUTE);
-        }
-        if ((mode & 0004) != 0) {
             permissions.add(PosixFilePermission.OTHERS_READ);
-        }
-        if ((mode & 0002) != 0) {
-            permissions.add(PosixFilePermission.OTHERS_WRITE);
-        }
-        if ((mode & 0001) != 0) {
             permissions.add(PosixFilePermission.OTHERS_EXECUTE);
+        } else {
+            // Mode 100644: rw-r--r-- (regular file)
+            permissions.add(PosixFilePermission.OWNER_READ);
+            permissions.add(PosixFilePermission.OWNER_WRITE);
+            permissions.add(PosixFilePermission.GROUP_READ);
+            permissions.add(PosixFilePermission.OTHERS_READ);
         }
         return permissions;
     }