The remaining index entries after replaced ones will be added to the
final index. These added entries are also sorted by entry name then
stage.
+
+== Untracked cache
+
+ Untracked cache saves the untracked file list and necessary data to
+ verify the cache. The signature for this extension is { 'U', 'N',
+ 'T', 'R' }.
+
+ The extension starts with
+
+ - Stat data of $GIT_DIR/info/exclude. See "Index entry" section from
+ ctime field until "file size".
+
+ - Stat data of core.excludesfile
+
+ - 32-bit dir_flags (see struct dir_struct)
+
+ - 160-bit SHA-1 of $GIT_DIR/info/exclude. Null SHA-1 means the file
+ does not exist.
+
+ - 160-bit SHA-1 of core.excludesfile. Null SHA-1 means the file does
+ not exist.
+
+ - NUL-terminated string of per-dir exclude file name. This usually
+ is ".gitignore".
+
+ - The number of following directory blocks, variable width
+ encoding. If this number is zero, the extension ends here with a
+ following NUL.
+
+ - A number of directory blocks in depth-first-search order, each
+ consists of
+
+ - The number of untracked entries, variable width encoding.
+
+ - The number of sub-directory blocks, variable width encoding.
+
+ - The directory name terminated by NUL.
+
+ - A number of untrached file/dir names terminated by NUL.
+
+The remaining data of each directory block is grouped by type:
+
+ - An ewah bitmap, the n-th bit marks whether the n-th directory has
+ valid untracked cache entries.
+
+ - An ewah bitmap, the n-th bit records "check-only" bit of
+ read_directory_recursive() for the n-th directory.
+
+ - An ewah bitmap, the n-th bit indicates whether SHA-1 and stat data
+ is valid for the n-th directory and exists in the next data.
+
+ - An array of stat data. The n-th data corresponds with the n-th
+ "one" bit in the previous ewah bitmap.
+
+ - An array of SHA-1. The n-th SHA-1 corresponds with the n-th "one" bit
+ in the previous ewah bitmap.
+
+ - One NUL.