[PATCH] commit: inform pre-commit if --amend is used

["Øystein Walle" <oystwa@gmail.com>]

When a commit is amended a pre-commit hook that verifies the commit's
contents might not find what it's looking for if for example it looks at
the differences against HEAD when HEAD~1 might be more appropriate.
Inform the commit hook that --amend is being used so that hook authors
can do e.g.

    if test "$1" = amend
    then
        ...
    else
        ...
    fi

to handle these situations.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
---
 Documentation/githooks.txt |  3 ++-
 builtin/commit.c           |  3 ++-
 t/t7503-pre-commit-hook.sh | 14 ++++++++++++++
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 9ef2469..e113027 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -73,7 +73,8 @@ pre-commit
 ~~~~~~~~~~
 
 This hook is invoked by 'git commit', and can be bypassed
-with `--no-verify` option.  It takes no parameter, and is
+with `--no-verify` option.  It takes one parameter which is "amend" if
+`--amend` was used when committing and empty otherwise. It 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.
diff --git a/builtin/commit.c b/builtin/commit.c
index e108c53..e38dd4a 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -694,7 +694,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	/* This checks and barfs if author is badly specified */
 	determine_author_info(author_ident);
 
-	if (!no_verify && run_commit_hook(use_editor, index_file, "pre-commit", NULL))
+	if (!no_verify && run_commit_hook(use_editor, index_file,
+					  "pre-commit", amend ? "amend" : "", NULL))
 		return 0;
 
 	if (squash_message) {
diff --git a/t/t7503-pre-commit-hook.sh b/t/t7503-pre-commit-hook.sh
index 984889b..be97676 100755
--- a/t/t7503-pre-commit-hook.sh
+++ b/t/t7503-pre-commit-hook.sh
@@ -136,4 +136,18 @@ test_expect_success 'check the author in hook' '
 	git show -s
 '
 
+# a hook that checks if "amend" is passed as an argument
+cat > "$HOOK" <<EOF
+#!/bin/sh
+test "\$1" = amend
+EOF
+
+test_expect_success 'check that "amend" argument is given' '
+       git commit --amend --allow-empty
+'
+
+test_expect_success 'check that "amend" argument is NOT given' '
+       ! git commit --allow-empty
+'
+
 test_done
-- 
2.0.3