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>

t/t0021-conversion.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+test_description='blob conversion via gitattributes'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	{
+	    echo "*.i ident"
+	} >.gitattributes &&
+
+	{
+	    echo a b c d e f g h i j k l m
+	    echo n o p q r s t u v w x y z
+	    echo '\''$ident$'\''
+	} >test &&
+	cat test >test.t &&
+	cat test >test.o &&
+	cat test >test.i &&
+	git add test test.t test.i &&
+	rm -f test test.t test.i &&
+	git checkout -- test test.t test.i
+'
+
+script='s/^\$ident: \([0-9a-f]*\) \$/\1/p'
+
+test_expect_success check '
+
+	cmp test.o test &&
+	cmp test.o test.t &&
+
+	# ident should be stripped in the repository
+	git diff --raw --exit-code :test :test.i &&
+	id=$(git rev-parse --verify :test) &&
+	embedded=$(sed -ne "$script" test.i) &&
+	test "z$id" = "z$embedded"
+'
+
+test_done