doc: clarify that --abbrev=<n> is about the minimum length

Early text written in 2006 explains the "--abbrev=<n>" option to
"show only a partial prefix", without saying that the length of the
partial prefix is not necessarily the number given to the option to
ensure that the output names the object uniquely.

Update documentation for the diff family of commands, "blame",
"branch --verbose", "ls-files" and "ls-tree" to stress that the
short prefix must uniquely refer to an object, and <n> is merely
the mininum number of hexdigits used in the prefix.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -446,7 +446,8 @@ endif::git-format-patch[]
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
 	name in diff-raw format output and diff-tree header
-	lines, show only a partial prefix.
+	lines, show the shortest prefix that is at least '<n>'
+	hexdigits long that uniquely refers the object.
 	In diff-patch output format, `--full-index` takes higher
 	precedence, i.e. if `--full-index` is specified, full blob
 	names will be shown regardless of `--abbrev`.

Documentation/git-blame.txt

@@ -87,7 +87,9 @@ include::blame-options.txt[]
 
 --abbrev=<n>::
 	Instead of using the default 7+1 hexadecimal digits as the
-	abbreviated object name, use <n>+1 digits. Note that 1 column
+	abbreviated object name, use <m>+1 digits, where <m> is at
+	least <n> but ensures the commit object names are unique.
+	Note that 1 column
 	is used for a caret to mark the boundary commit.
 
 

Documentation/git-branch.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git branch' [--color[=<when>] | --no-color] [--show-current]
-	[-v [--abbrev=<length> | --no-abbrev]]
+	[-v [--abbrev=<n> | --no-abbrev]]
 	[--column[=<options>] | --no-column] [--sort=<key>]
 	[--merged [<commit>]] [--no-merged [<commit>]]
 	[--contains [<commit>]] [--no-contains [<commit>]]
@@ -194,8 +194,10 @@ This option is only applicable in non-verbose mode.
 	Be more quiet when creating or deleting a branch, suppressing
 	non-error messages.
 
---abbrev=<length>::
-	Alter the sha1's minimum display length in the output listing.
+--abbrev=<n>::
+	In the verbose listing that show the commit object name,
+	show the shortest prefix that is at least '<n>' hexdigits
+	long that uniquely refers the object.
 	The default value is 7 and can be overridden by the `core.abbrev`
 	config option.
 

Documentation/git-ls-files.txt

@@ -19,7 +19,7 @@ SYNOPSIS
 		[--exclude-standard]
 		[--error-unmatch] [--with-tree=<tree-ish>]
 		[--full-name] [--recurse-submodules]
-		[--abbrev] [--] [<file>...]
+		[--abbrev[=<n>]] [--] [<file>...]
 
 DESCRIPTION
 -----------
@@ -153,7 +153,8 @@ a space) at the start of each line:
 
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
-	lines, show only a partial prefix.
+	lines, show the shortest prefix that is at least '<n>'
+	hexdigits long that uniquely refers the object.
 	Non default number of digits can be specified with --abbrev=<n>.
 
 --debug::

Documentation/git-ls-tree.txt

@@ -62,7 +62,8 @@ OPTIONS
 
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
-	lines, show only a partial prefix.
+	lines, show the shortest prefix that is at least '<n>'
+	hexdigits long that uniquely refers the object.
 	Non default number of digits can be specified with --abbrev=<n>.
 
 --full-name::

Documentation/pretty-options.txt

@@ -16,9 +16,9 @@ configuration (see linkgit:git-config[1]).
 
 --abbrev-commit::
 	Instead of showing the full 40-byte hexadecimal commit object
-	name, show only a partial prefix.  Non default number of
-	digits can be specified with "--abbrev=<n>" (which also modifies
-	diff output, if it is displayed).
+	name, show a prefix that names the object uniquely.
+	"--abbrev=<n>" (which also modifies diff output, if it is displayed)
+	option can be used to specify the minimum length of the prefix.
 +
 This should make "--pretty=oneline" a whole lot more readable for
 people using 80-column terminals.