merge: teach --autostash option

In rebase, one can pass the `--autostash` option to cause the worktree
to be automatically stashed before continuing with the rebase. This
option is missing in merge, however.

Implement the `--autostash` option and corresponding `merge.autoStash`
option in merge which stashes before merging and then pops after.

This option is useful when a developer has some local changes on a topic
branch but they realize that their work depends on another branch.
Previously, they had to run something like

	git fetch ...
	git stash push
	git merge FETCH_HEAD
	git stash pop

but now, that is reduced to

	git fetch ...
	git merge --autostash FETCH_HEAD

When an autostash is generated, it is automatically reapplied to the
worktree only in three explicit situations:

	1. An incomplete merge is commit using `git commit`.
	2. A merge completes successfully.
	3. A merge is aborted using `git merge --abort`.

In all other situations where the merge state is removed using
remove_merge_branch_state() such as aborting a merge via
`git reset --hard`, the autostash is saved into the stash reflog
instead keeping the worktree clean.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Suggested-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/parse-options.h b/parse-options.h
index fece5ba628..46af942093 100644
--- a/parse-options.h
+++ b/parse-options.h
@@ -336,5 +336,6 @@ int parse_opt_passthru_argv(const struct option *, const char *, int);
 #define OPT_CLEANUP(v) OPT_STRING(0, "cleanup", v, N_("mode"), N_("how to strip spaces and #comments from message"))
 #define OPT_PATHSPEC_FROM_FILE(v) OPT_FILENAME(0, "pathspec-from-file", v, N_("read pathspec from file"))
 #define OPT_PATHSPEC_FILE_NUL(v)  OPT_BOOL(0, "pathspec-file-nul", v, N_("with --pathspec-from-file, pathspec elements are separated with NUL character"))
+#define OPT_AUTOSTASH(v) OPT_BOOL(0, "autostash", v, N_("automatically stash/stash pop before and after"))
 
 #endif