[PATCH 1/2 v2] commit.c - provide base commit to the hooks/pre-commit script

[Mark Levedahl <mlevedahl@gmail.com>]

If hooks/pre-commit acts based upon the changes to be checked in
rather than just the resulting content, the script needs to know which
commit to use. For a normal commit, this is HEAD, but when amending this
is HEAD~1. So, this modifies commit.c to pass $1 as HEAD|HEAD~1 depending
upon the commit type. Existing scripts are unaffected as they did not
expect any argument so will silently ignore this extra bit of info.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
---
 Documentation/githooks.txt         |   10 ++++++----
 builtin/commit.c                   |    2 +-
 templates/hooks--pre-commit.sample |    9 +++++----
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 7183aa9..d785b6c 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -73,10 +73,12 @@ pre-commit
 ~~~~~~~~~~
 
 This hook is invoked by 'git commit', and can be bypassed
-with `\--no-verify` option.  It takes no parameter, and is
-invoked before obtaining the proposed commit log message and
-making a commit.  Exiting with non-zero status from this script
-causes the 'git commit' to abort.
+with `\--no-verify` option.  It takes a single parameter naming
+the current commit's parent (HEAD | HEAD~1), and is invoked before
+obtaining the proposed commit log message and making a commit.
+Note that $1=HEAD~1 indicates an amended commit is in process.
+Exiting with non-zero status from this script causes the 'git
+commit' to abort.
 
 The default 'pre-commit' hook, when enabled, catches introduction
 of lines with trailing whitespaces and aborts the commit when
diff --git a/builtin/commit.c b/builtin/commit.c
index ddf77e4..49f799e 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -544,7 +544,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	const char *hook_arg2 = NULL;
 	int ident_shown = 0;
 
-	if (!no_verify && run_hook(index_file, "pre-commit", NULL))
+	if (!no_verify && run_hook(index_file, "pre-commit", amend ? "HEAD~1" : "HEAD", NULL))
 		return 0;
 
 	if (message.len) {
diff --git a/templates/hooks--pre-commit.sample b/templates/hooks--pre-commit.sample
index b187c4b..e72cf9c 100755
--- a/templates/hooks--pre-commit.sample
+++ b/templates/hooks--pre-commit.sample
@@ -1,15 +1,16 @@
 #!/bin/sh
 #
 # An example hook script to verify what is about to be committed.
-# Called by "git commit" with no arguments.  The hook should
-# exit with non-zero status after issuing an appropriate message if
-# it wants to stop the commit.
+# Called by "git commit" with a single argument indicating the
+# commit-base (HEAD|HEAD~1).  The hook should exit with non-zero
+# status after issuing an appropriate message if it wants to stop
+# the commit.
 #
 # To enable this hook, rename this file to "pre-commit".
 
 if git rev-parse --verify HEAD >/dev/null 2>&1
 then
-	against=HEAD
+	against=$1
 else
 	# Initial commit: diff against an empty tree object
 	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
-- 
1.7.1.269.g9fb6