Add 'ident' conversion.

The 'ident' attribute set to path squashes "$ident:<any bytes
except dollor sign>$" to "$ident$" upon checkin, and expands it
to "$ident: <blob SHA-1> $" upon checkout.

As we have two conversions that affect checkin/checkout paths,
clarify how they interact with each other.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/gitattributes.txt

@@ -78,12 +78,17 @@ are attributes-aware.
 Checking-out and checking-in
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The attribute `crlf` affects how the contents stored in the
+These attributes affect how the contents stored in the
 repository are copied to the working tree files when commands
-such as `git checkout` and `git merge` run.  It also affects how
+such as `git checkout` and `git merge` run.  They also affect how
 git stores the contents you prepare in the working tree in the
 repository upon `git add` and `git commit`.
 
+`crlf`
+^^^^^^
+
+This attribute controls the line-ending convention.
+
 Set::
 
 	Setting the `crlf` attribute on a path is meant to mark
@@ -129,6 +134,28 @@ converted to LF upon checkin, but there is no conversion done
 upon checkout.
 
 
+`ident`
+^^^^^^^
+
+When the attribute `ident` is set to a path, git replaces
+`$ident$` in the blob object with `$ident:`, followed by
+40-character hexadecimal blob object name, followed by a dollar
+sign `$` upon checkout.  Any byte sequence that begins with
+`$ident:` and ends with `$` in the worktree file is replaced
+with `$ident$` upon check-in.
+
+
+Interaction between checkin/checkout attributes
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In the check-in codepath, the worktree file is first converted
+with `ident` (if specified), and then with `crlf` (again, if
+specified and applicable).
+
+In the check-out codepath, the blob content is first converted
+with `crlf`, and then `ident`.
+
+
 Generating diff text
 ~~~~~~~~~~~~~~~~~~~~