user-manual: introduce the word "commit" earlier

Use the word "commit" as a synonym for "version" from the start.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -66,14 +66,14 @@ contains all the information about the history of the project.
 How to check out a different version of a project
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-Git is best thought of as a tool for storing the history of a
-collection of files.  It stores the history as a compressed
-collection of interrelated snapshots (versions) of the project's
-contents.
+Git is best thought of as a tool for storing the history of a collection
+of files.  It stores the history as a compressed collection of
+interrelated snapshots of the project's contents.  In git each such
+version is called a <<def_commit,commit>>.
 
 A single git repository may contain multiple branches.  It keeps track
 of them by keeping a list of <<def_head,heads>> which reference the
-latest version on each branch; the gitlink:git-branch[1] command shows
+latest commit on each branch; the gitlink:git-branch[1] command shows
 you the list of branch heads:
 
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