user-manual: rewrite index discussion

Add an example using git-ls-files, standardize on the new "index"
terminology (as opposed to "cache"), attempt to clarify discussion and
make it a little shorter, avoid some unnecessary jargon ("write-back
cache").

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -2911,57 +2911,63 @@ gitlink:git-verify-tag[1].
 
 
 [[the-index]]
-The "index" aka "Current Directory Cache"
------------------------------------------
+The index
+-----------
 
-The index is a simple binary file, which contains an efficient
-representation of the contents of a virtual directory.  It
-does so by a simple array that associates a set of names, dates,
-permissions and content (aka "blob") objects together.  The cache is
-always kept ordered by name, and names are unique (with a few very
-specific rules) at any point in time, but the cache has no long-term
-meaning, and can be partially updated at any time.
+The index is a binary file (generally kept in .git/index) containing a
+sorted list of path names, each with permissions and the SHA1 of a blob
+object; gitlink:git-ls-files[1] can show you the contents of the index:
 
-In particular, the index certainly does not need to be consistent with
-the current directory contents (in fact, most operations will depend on
-different ways to make the index 'not' be consistent with the directory
-hierarchy), but it has three very important attributes:
+-------------------------------------------------
+$ git ls-files --stage
+100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0	.gitignore
+100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0	.mailmap
+100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0	COPYING
+100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0	Documentation/.gitignore
+100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0	Documentation/Makefile
+...
+100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0	xdiff/xtypes.h
+100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0	xdiff/xutils.c
+100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0	xdiff/xutils.h
+-------------------------------------------------
 
-'(a) it can re-generate the full state it caches (not just the
-directory structure: it contains pointers to the "blob" objects so
-that it can regenerate the data too)'
+Note that in older documentation you may see the index called the
+"current directory cache" or just the "cache".  It has three important
+properties:
 
-As a special case, there is a clear and unambiguous one-way mapping
-from a current directory cache to a "tree object", which can be
-efficiently created from just the current directory cache without
-actually looking at any other data.  So a directory cache at any one
-time uniquely specifies one and only one "tree" object (but has
-additional data to make it easy to match up that tree object with what
-has happened in the directory)
+1. The index contains all the information necessary to generate a single
+(uniquely determined) tree object.
++
+For example, running gitlink:git-commit[1] generates this tree object
+from the index, stores it in the object database, and uses it as the
+tree object associated with the new commit.
 
-'(b) it has efficient methods for finding inconsistencies between that
-cached state ("tree object waiting to be instantiated") and the
-current state.'
+2. The index enables fast comparisons between the tree object it defines
+and the working tree.
++
+It does this by storing some additional data for each entry (such as
+the last modified time).  This data is not displayed above, and is not
+stored in the created tree object, but it can be used to determine
+quickly which files in the working directory differ from what was
+stored in the index, and thus save git from having to read all of the
+data from such files to look for changes.
 
-'(c) it can additionally efficiently represent information about merge
-conflicts between different tree objects, allowing each pathname to be
+3. It can efficiently represent information about merge conflicts
+between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
-you can create a three-way merge between them.'
+you can create a three-way merge between them.
++
+We saw in <<conflict-resolution>> that during a merge the index can
+store multiple versions of a single file (called "stages").  The third
+column in the gitlink:git-ls-files[1] output above is the stage
+number, and will take on values other than 0 for files with merge
+conflicts.
 
-Those are the ONLY three things that the directory cache does.  It's a
-cache, and the normal operation is to re-generate it completely from a
-known tree object, or update/compare it with a live tree that is being
-developed.  If you blow the directory cache away entirely, you generally
-haven't lost any information as long as you have the name of the tree
-that it described.
+The index is thus a sort of temporary staging area, which is filled with
+a tree which you are in the process of working on.
 
-At the same time, the index is also the staging area for creating
-new trees, and creating a new tree always involves a controlled
-modification of the index file.  In particular, the index file can
-have the representation of an intermediate tree that has not yet been
-instantiated.  So the index can be thought of as a write-back cache,
-which can contain dirty information that has not yet been written back
-to the backing store.
+If you blow the index away entirely, you generally haven't lost any
+information as long as you have the name of the tree that it described.
 
 [[low-level-operations]]
 Low-level git operations