[PATCH/RFC] commit: new option to abort -a something is already staged

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

So many times I have carefully cherry picked changes to the index with
`git add -p` then accidentally did `git commit -am ....` (usually by
retrieving a command from history and pressing Enter too quickly)
which destroyed beautiful index.

One way to deal with this is some form of `git undo` that allows me to
retrieve the old index. That's not a lot of work by itself. The problem
is designing that `git undo` interface because there are more undo
options that this.

Instead, let's handle just this problem for now. This new option
commit.preciousDirtyIndex, if set to false, will not allow `commit -a`
to continue if the final index is different from the existing one. I
don't think this can be achieved with hooks because the hooks don't
know about "-a" or different commit modes.

Or is there a better way to handle this?

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/config.txt |  5 +++++
 builtin/commit.c         | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 95ad715a44..3937681ee9 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1417,6 +1417,11 @@ commit.gpgSign::
 	convenient to use an agent to avoid typing your GPG passphrase
 	several times.
 
+commit.preciousDirtyIndex::
+	If some changes are partially staged in the index (i.e.
+	"git commit -a" and "git commit" commit different changes), reject
+	"git commit -a".
+
 commit.status::
 	A boolean to enable/disable inclusion of status information in the
 	commit message template when using an editor to prepare the commit
diff --git a/builtin/commit.c b/builtin/commit.c
index 213fca2d8e..489e4b9f50 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -98,6 +98,7 @@ static const char *author_message, *author_message_buffer;
 static char *edit_message, *use_message;
 static char *fixup_message, *squash_message;
 static int all, also, interactive, patch_interactive, only, amend, signoff;
+static int allow_dirty_index = 1;
 static int edit_flag = -1; /* unspecified */
 static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static int config_commit_verbose = -1; /* unspecified */
@@ -385,10 +386,24 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
 	 * (B) on failure, rollback the real index.
 	 */
 	if (all || (also && pathspec.nr)) {
+		int compare_oid = all && !allow_dirty_index;
+		struct object_id previous_oid;
+
+		if (compare_oid) {
+			if (update_main_cache_tree(0) || !the_index.cache_tree)
+				die(_("error building trees"));
+			if (the_index.cache_tree->entry_count >= 0)
+				oidcpy(&previous_oid, &the_index.cache_tree->oid);
+			else
+				oidclr(&previous_oid);
+		}
 		hold_locked_index(&index_lock, LOCK_DIE_ON_ERROR);
 		add_files_to_cache(also ? prefix : NULL, &pathspec, 0);
 		refresh_cache_or_die(refresh_flags);
 		update_main_cache_tree(WRITE_TREE_SILENT);
+		if (compare_oid && the_index.cache_tree &&
+		    oidcmp(&previous_oid, &the_index.cache_tree->oid))
+			die(_("staged content is different, aborting"));
 		if (write_locked_index(&the_index, &index_lock, 0))
 			die(_("unable to write new_index file"));
 		commit_style = COMMIT_NORMAL;
@@ -1413,6 +1428,10 @@ static int git_commit_config(const char *k, const char *v, void *cb)
 		config_commit_verbose = git_config_bool_or_int(k, v, &is_bool);
 		return 0;
 	}
+	if (!strcmp(k, "commit.preciousdirtyindex")) {
+		allow_dirty_index = !git_config_bool(k, v);
+		return 0;
+	}
 
 	status = git_gpg_config(k, v, NULL);
 	if (status)
-- 
2.18.0.1003.g5e2e2c8169