glossary: add definition for overlay

Add a definition for what overlay means in the context of git, to
clarify the recently introduced overlay-mode in git checkout.

Helped-by: Elijah Newren <newren@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/glossary-content.txt b/Documentation/glossary-content.txt
index 0d2aa48c63856aba78d559445186a9b9e6bddd7d..d7fb770cf5de6729f7fb973903183132139f79ac 100644 (file)
--- a/Documentation/glossary-content.txt
+++ b/Documentation/glossary-content.txt
@@ -287,6 +287,15 @@ This commit is referred to as a "merge commit", or sometimes just a
        origin/name-of-upstream-branch, which you can see using
        `git branch -r`.
 
+[[def_overlay]]overlay::
+       Only update and add files to the working directory, but don't
+       delete them, similar to how 'cp -R' would update the contents
+       in the destination directory.  This is the default mode in a
+       <<def_checkout,checkout>> when checking out files from the
+       <<def_index,index>> or a <<def_tree-ish,tree-ish>>.  In
+       contrast, no-overlay mode also deletes tracked files not
+       present in the source, similar to 'rsync --delete'.
+
 [[def_pack]]pack::
        A set of objects which have been compressed into one file (to save space
        or to transmit them efficiently).