gitcli: describe abbreviation of long options

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/gitcli.txt

@@ -62,6 +62,14 @@ scripting git:
    `git log -1 HEAD` but write `git log -1 HEAD --`; the former will not work
    if you happen to have a file called `HEAD` in the work tree.
 
+ * many commands allow a long option "--option" to be abbreviated
+   only to their unique prefix (e.g. if there is no other option
+   whose name begins with "opt", you may be able to spell "--opt" to
+   invoke the "--option" flag), but you should fully spell them out
+   when writing your scripts; later versions of Git may introduce a
+   new option whose name shares the same prefix, e.g. "--optimize",
+   to make a short prefix that used to be unique no longer unique.
+
 
 ENHANCED OPTION PARSER
 ----------------------