Merge branch 'maint-1.8.1' into maint

* maint-1.8.1:
  bundle: Add colons to list headings in "verify"
  bundle: Fix "verify" output if history is complete
  Documentation: filter-branch env-filter example
  git-filter-branch.txt: clarify ident variables usage
  git-compat-util.h: Provide missing netdb.h definitions
  describe: Document --match pattern format
  Documentation/githooks: Explain pre-rebase parameters
  update-index: list supported idx versions and their features
  diff-options: unconfuse description of --color
  read-cache.c: use INDEX_FORMAT_{LB,UB} in verify_hdr()
  index-format.txt: mention of v4 is missing in some places

Documentation/git-update-index.txt

@@ -145,7 +145,15 @@ you will need to handle the situation manually.
 
 --index-version <n>::
 	Write the resulting index out in the named on-disk format version.
-	The current default version is 2.
+	Supported versions are 2, 3 and 4. The current default version is 2
+	or 3, depending on whether extra features are used, such as
+	`git add -N`.
++
+Version 4 performs a simple pathname compression that reduces index
+size by 30%-50% on large repositories, which results in faster load
+time. Version 4 is relatively young (first released in in 1.8.0 in
+October 2012). Other Git implementations such as JGit and libgit2
+may not support it yet.
 
 -z::
 	Only meaningful with `--stdin` or `--index-info`; paths are