Merge branch 'ty/doc-diff-u-wo-number'

"git diff -U<num>" was too lenient in its command line parsing and
took an empty string as a valid <num>.

* ty/doc-diff-u-wo-number:
  diff: document -U without <n> as using default context

Author: gitster

Documentation/diff-context-options.adoc

@@ -1,7 +1,9 @@
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context. Defaults to `diff.context`
-	or 3 if the config option is unset.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the configuration variable
+	is unset. (`-U` without `<n>` is silently accepted as a synonym for
+	`-p` due to a historical accident).
 
 `--inter-hunk-context=<n>`::
 	Show the context between diff hunks, up to the specified _<number>_

Documentation/diff-options.adoc

@@ -127,8 +127,10 @@ endif::git-log[]
 
 `-U<n>`::
 `--unified=<n>`::
-	Generate diffs with _<n>_ lines of context instead of
-	the usual three.
+	Generate diffs with _<n>_ lines of context. The number of context
+	lines defaults to `diff.context` or 3 if the configuration variable
+	is unset. (`-U` without `<n>` is silently accepted as a synonym for
+	`-p` due to a historical accident).
 ifndef::git-format-patch[]
 	Implies `--patch`.
 endif::git-format-patch[]