git-apply: add --allow-empty flag

Some users or scripts will pipe "git diff"
output to "git apply" when replaying diffs
or commits. In these cases, they will rely
on the return value of "git apply" to know
whether the diff was applied successfully.

However, for empty commits, "git apply" will
fail. This complicates scripts since they
have to either buffer the diff and check
its length, or run diff again with "exit-code",
essentially doing the diff twice.

Add the "--allow-empty" flag to "git apply"
which allows it to handle both empty diffs
and empty commits created by "git format-patch
--always" by doing nothing and returning 0.

Add tests for both with and without --allow-empty.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 	  [--ignore-space-change | --ignore-whitespace]
 	  [--whitespace=(nowarn|warn|fix|error|error-all)]
 	  [--exclude=<path>] [--include=<path>] [--directory=<root>]
-	  [--verbose | --quiet] [--unsafe-paths] [<patch>...]
+	  [--verbose | --quiet] [--unsafe-paths] [--allow-empty] [<patch>...]
 
 DESCRIPTION
 -----------
@@ -256,6 +256,10 @@ When `git apply` is used as a "better GNU patch", the user can pass
 the `--unsafe-paths` option to override this safety check.  This option
 has no effect when `--index` or `--cached` is in use.
 
+--allow-empty::
+	Don't return error for patches containing no diff. This includes
+	empty patches and patches with commit text only.
+
 CONFIGURATION
 -------------