More precise description of 'git describe --abbrev'

Also adds a note about why the output in the examples might give
different output today.

Signed-off-by: Gisle Aas <gisle@aas.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -44,7 +44,9 @@ OPTIONS
 
 --abbrev=<n>::
 	Instead of using the default 7 hexadecimal digits as the
-	abbreviated object name, use <n> digits.
+	abbreviated object name, use <n> digits, or as many digits
+	as needed to form a unique object name.  An <n> of 0
+	will suppress long format, only showing the closest tag.
 
 --candidates=<n>::
 	Instead of considering only the 10 most recent tags as
@@ -68,8 +70,8 @@ OPTIONS
 	This is useful when you want to see parts of the commit object name
 	in "describe" output, even when the commit in question happens to be
 	a tagged version.  Instead of just emitting the tag name, it will
-	describe such a commit as v1.2-0-deadbeef (0th commit since tag v1.2
+	describe such a commit as v1.2-0-gdeadbee (0th commit since tag v1.2
-	that points at object deadbeef....).
+	that points at object deadbee....).
 
 --match <pattern>::
 	Only consider tags matching the given pattern (can be used to avoid
@@ -108,7 +110,7 @@ the output shows the reference path as well:
 	[torvalds@g5 git]$ git describe --all --abbrev=4 v1.0.5^2
 	tags/v1.0.0-21-g975b
 
-	[torvalds@g5 git]$ git describe --all HEAD^
+	[torvalds@g5 git]$ git describe --all --abbrev=4 HEAD^
 	heads/lt/describe-7-g975b
 
 With --abbrev set to 0, the command can be used to find the
@@ -117,6 +119,13 @@ closest tagname without any suffix:
 	[torvalds@g5 git]$ git describe --abbrev=0 v1.0.5^2
 	tags/v1.0.0
 
+Note that the suffix you get if you type these commands today may be
+longer than what Linus saw above when he ran this command, as your
+git repository may have new commits whose object names begin with
+975b that did not exist back then, and "-g975b" suffix alone may not
+be sufficient to disambiguate these commits.
+
+
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