[PATCH] git-patch-id.txt: show that you can pipe in git-log

Change the manual page for git-patch-id(1) to mention in the SYNOPSIS
that you can pipe in the git-log(1) output.

The manual page only mentioned that you could pipe in `< patch`, I had
to grep the test suite to find that you could do things like `git log
-p -1 | git patch-id`.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---
 Documentation/git-patch-id.txt |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
index 4dae139..fec2cf3 100644
--- a/Documentation/git-patch-id.txt
+++ b/Documentation/git-patch-id.txt

@@ -9,6 +9,8 @@ SYNOPSIS
 --------
 'git patch-id' < <patch>
 
+'git log --pretty=format:%H -p | git patch-id'
+
 DESCRIPTION
 -----------
 A "patch ID" is nothing but a SHA1 of the diff associated with a patch, with

-- 
1.7.2.3