Change 'cache' to 'index' in the docs

This patch makes the documentation refer to the index
as index instead of cache, but some references still
remain. (e.g. git-update-index.txt)

Signed-off-by: Lukas Sandström <lukass@etek.chalmers.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/Documentation/git-merge-index.txt b/Documentation/git-merge-index.txt
index d072fda..6030642 100644
--- a/Documentation/git-merge-index.txt
+++ b/Documentation/git-merge-index.txt
@@ -12,7 +12,7 @@
 
 DESCRIPTION
 -----------
-This looks up the <file>(s) in the cache and, if there are any merge
+This looks up the <file>(s) in the index and, if there are any merge
 entries, passes the SHA1 hash for those files as arguments 1, 2, 3 (empty
 argument if no file), and <file> as argument 4.  File modes for the three
 files are passed as arguments 5, 6 and 7.
@@ -23,7 +23,7 @@
 	Interpret all following arguments as filenames.
 
 -a::
-	Run merge against all files in the cache that need merging.
+	Run merge against all files in the index that need merging.
 
 -o::
 	Instead of stopping at the first failed merge, do all of them