glossary: add definitions for dereference & peel

Add 'gitglossary' definitions for "dereference" (as it used for both symrefs
and objects) and "peel". These terms are used in options and documentation
throughout Git, but they are not clearly defined anywhere and the behavior
they refer to depends heavily on context. Provide explicit definitions to
clarify existing documentation to users and help contributors to use the
most appropriate terminology possible in their additions to Git.

Update other definitions in the glossary that use the term "dereference" to
link to 'def_dereference'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -98,9 +98,8 @@ to point at the new commit.
 	revision.
 
 [[def_commit-ish]]commit-ish (also committish)::
-	A <<def_commit_object,commit object>> or an
-	<<def_object,object>> that can be recursively dereferenced to
-	a commit object.
+	A <<def_commit_object,commit object>> or an <<def_object,object>> that
+	can be recursively <<def_dereference,dereferenced>> to a commit object.
 	The following are all commit-ishes:
 	a commit object,
 	a <<def_tag_object,tag object>> that points to a commit
@@ -125,6 +124,25 @@ to point at the new commit.
 	dangling object has no references to it from any
 	reference or <<def_object,object>> in the <<def_repository,repository>>.
 
+[[def_dereference]]dereference::
+	Referring to a <<def_symref,symbolic ref>>: the action of accessing the
+	<<def_ref,reference>> pointed at by a symbolic ref. Recursive
+	dereferencing involves repeating the aforementioned process on the
+	resulting ref until a non-symbolic reference is found.
++
+Referring to a <<def_tag_object,tag object>>: the action of accessing the
+<<def_object,object>> a tag points at. Tags are recursively dereferenced by
+repeating the operation on the result object until the result has either a
+specified <<def_object_type,object type>> (where applicable) or any non-"tag"
+object type. A synonym for "recursive dereference" in the context of tags is
+"<<def_peel,peel>>".
++
+Referring to a <<def_commit_object,commit object>>: the action of accessing
+the commit's tree object. Commits cannot be dereferenced recursively.
++
+Unless otherwise specified, "dereferencing" as it used in the context of Git
+commands or protocols is implicitly recursive.
+
 [[def_detached_HEAD]]detached HEAD::
 	Normally the <<def_HEAD,HEAD>> stores the name of a
 	<<def_branch,branch>>, and commands that operate on the
@@ -444,6 +462,10 @@ exclude;;
 	of the logical predecessor(s) in the line of development, i.e. its
 	parents.
 
+[[def_peel]]peel::
+	The action of recursively <<def_dereference,dereferencing>> a
+	<<def_tag_object,tag object>>.
+
 [[def_pickaxe]]pickaxe::
 	The term <<def_pickaxe,pickaxe>> refers to an option to the diffcore
 	routines that help select changes that add or delete a given text
@@ -620,12 +642,11 @@ The most notable example is `HEAD`.
 	copies of) commit objects of the contained submodules.
 
 [[def_symref]]symref::
-	Symbolic reference: instead of containing the <<def_SHA1,SHA-1>>
-	id itself, it is of the format 'ref: refs/some/thing' and when
-	referenced, it recursively dereferences to this reference.
-	'<<def_HEAD,HEAD>>' is a prime example of a symref. Symbolic
-	references are manipulated with the linkgit:git-symbolic-ref[1]
-	command.
+	Symbolic reference: instead of containing the <<def_SHA1,SHA-1>> id
+	itself, it is of the format 'ref: refs/some/thing' and when referenced,
+	it recursively <<def_dereference,dereferences>> to this reference.
+	'<<def_HEAD,HEAD>>' is a prime example of a symref. Symbolic references
+	are manipulated with the linkgit:git-symbolic-ref[1] command.
 
 [[def_tag]]tag::
 	A <<def_ref,ref>> under `refs/tags/` namespace that points to an
@@ -661,11 +682,11 @@ The most notable example is `HEAD`.
 	<<def_tree,tree>> is equivalent to a <<def_directory,directory>>.
 
 [[def_tree-ish]]tree-ish (also treeish)::
-	A <<def_tree_object,tree object>> or an <<def_object,object>>
-	that can be recursively dereferenced to a tree object.
-	Dereferencing a <<def_commit_object,commit object>> yields the
-	tree object corresponding to the <<def_revision,revision>>'s
-	top <<def_directory,directory>>.
+	A <<def_tree_object,tree object>> or an <<def_object,object>> that can
+	be recursively <<def_dereference,dereferenced>> to a tree object.
+	Dereferencing a <<def_commit_object,commit object>> yields the tree
+	object corresponding to the <<def_revision,revision>>'s top
+	<<def_directory,directory>>.
 	The following are all tree-ishes:
 	a <<def_commit-ish,commit-ish>>,
 	a tree object,