Merge branch 'pb/prepare-commit-msg'

* pb/prepare-commit-msg:
  git-commit: add a prepare-commit-msg hook
  git-commit: Refactor creation of log message.
  git-commit: set GIT_EDITOR=: if editor will not be launched
  git-commit: support variable number of hook arguments

Documentation/git-commit.txt

@@ -280,8 +280,8 @@ order).
 
 HOOKS
 -----
-This command can run `commit-msg`, `pre-commit`, and
-`post-commit` hooks.  See link:hooks.html[hooks] for more
+This command can run `commit-msg`, `prepare-commit-msg`, `pre-commit`,
+and `post-commit` hooks.  See link:hooks.html[hooks] for more
 information.
 
 

Documentation/hooks.txt

@@ -61,6 +61,35 @@ The default 'pre-commit' hook, when enabled, catches introduction
 of lines with trailing whitespaces and aborts the commit when
 such a line is found.
 
+All the `git-commit` hooks are invoked with the environment
+variable `GIT_EDITOR=:` if the command will not bring up an editor
+to modify the commit message.
+
+prepare-commit-msg
+------------------
+
+This hook is invoked by `git-commit` right after preparing the
+default log message, and before the editor is started.
+
+It takes one to three parameters.  The first is the name of the file
+that the commit log message.  The second is the source of the commit
+message, and can be: `message` (if a `\-m` or `\-F` option was
+given); `template` (if a `\-t` option was given or the
+configuration option `commit.template` is set); `merge` (if the
+commit is a merge or a `.git/MERGE_MSG` file exists); `squash`
+(if a `.git/SQUASH_MSG` file exists); or `commit`, followed by
+a commit SHA1 (if a `\-c`, `\-C` or `\--amend` option was given).
+
+If the exit status is non-zero, `git-commit` will abort.
+
+The purpose of the hook is to edit the message file in place, and
+it is not suppressed by the `\--no-verify` option.  A non-zero exit
+means a failure of the hook and aborts the commit.  It should not
+be used as replacement for pre-commit hook.
+
+The sample `prepare-commit-msg` hook that comes with git comments
+out the `Conflicts:` part of a merge's commit message.
+
 commit-msg
 ----------