ref-format documentation.

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

.gitignore

@@ -8,6 +8,7 @@ git-archimport
 git-bisect
 git-branch
 git-cat-file
+git-check-ref-format
 git-checkout
 git-checkout-index
 git-cherry

Documentation/git-check-ref-format.txt

@@ -0,0 +1,50 @@
+git-check-ref-format(1)
+=======================
+
+NAME
+----
+git-check-ref-format - Make sure ref name is well formed.
+
+SYNOPSIS
+--------
+'git-check-ref-format' <refname>
+
+DESCRIPTION
+-----------
+Checks if a given 'refname' is acceptable, and exits non-zero if
+it is not.
+
+A reference is used in git to specify branches and tags.  A
+branch head is stored under `$GIT_DIR/refs/heads` directory, and
+a tag is stored under `$GIT_DIR/refs/tags` directory.  git
+imposes the following rules on how refs are named:
+
+. It could be named hierarchically (i.e. separated with slash
+  `/`), but each of its component cannot begin with a dot `.`;
+
+. It cannot have two consecutive dots `..` anywhere;
+
+. It cannot have ASCII control character (i.e. bytes whose
+  values are lower than \040, or \177 `DEL`), space, tilde `~`,
+  caret `{caret}`, or colon `:` anywhere;
+
+. It cannot end with a slash `/`.
+
+These rules makes it easy for shell script based tools to parse
+refnames, and also avoids ambiguities in certain refname
+expressions (see gitlink:git-rev-parse[1]).  Namely:
+
+. double-dot `..` are often used as in `ref1..ref2`, and in some
+  context this notation means `{caret}ref1 ref2` (i.e. not in
+  ref1 and in ref2).
+
+. tilde `~` and caret `{caret}` are used to introduce postfix
+  'nth parent' and 'peel onion' operation.
+
+. colon `:` is used as in `srcref:dstref` to mean "use srcref\'s
+  value and store it in dstref" in fetch and push operations.
+
+
+GIT
+---
+Part of the gitlink:git[7] suite

Documentation/git-rev-parse.txt

@@ -79,8 +79,9 @@ OPTIONS
 SPECIFYING REVISIONS
 --------------------
 
-A revision parameter typically names a commit object.  They use
-what is called an 'extended SHA1' syntax.
+A revision parameter typically, but not necessarily, names a
+commit object.  They use what is called an 'extended SHA1'
+syntax.
 
 * The full SHA1 object name (40-byte hexadecimal string), or
   a substring of such that is unique within the repository.
@@ -106,6 +107,18 @@ what is called an 'extended SHA1' syntax.
   equivalent to rev{caret}{caret}{caret} which is equivalent to\
   rev{caret}1{caret}1{caret}1.
 
+* A suffix '{caret}' followed by an object type name enclosed in
+  brace pair (e.g. `v0.99.8{caret}\{commit\}`) means the object
+  could be a tag, and dereference the tag recursively until an
+  object of that type is found or the object cannot be
+  dereferenced anymore (in which case, barf).  `rev{caret}0`
+  introduced earlier is a short-hand for `rev{caret}\{commit\}`.
+
+* A suffix '{caret}' followed by an empty brace pair
+  (e.g. `v0.99.8{caret}\{\}`) means the object could be a tag,
+  and dereference the tag recursively until a non-tag object is
+  found.
+
 'git-rev-parse' also accepts a prefix '{caret}' to revision parameter,
 which is passed to 'git-rev-list'.  Two revision parameters
 concatenated with '..' is a short-hand for writing a range