[PATCH 3/4] merge: GIT_MERGE_ALLOW_UNRELATED_HISTORIES environment

[Junio C Hamano <gitster@pobox.com>]

It is rumored that some scripts rely on being able to regularly
create two project merges.  Instead of forcing them to pass the
option --allow-unrelated-histories when they call "git merge", allow
them to set and export an environment at the beginning and forget
about it.

This will be less damaging than adding a configuration variable to
disable the safety, as contaminating the configuration file of users
of such a script will allow any invocation of "git merge", not
limited to such a script, to go without the safety.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git.txt | 3 +++
 builtin/merge.c       | 3 +++
 builtin/pull.c        | 7 ++++++-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/Documentation/git.txt b/Documentation/git.txt
index 754dc80..5c9380d 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -1138,6 +1138,9 @@ of clones and fetches.
 	  - any external helpers are named by their protocol (e.g., use
 	    `hg` to allow the `git-remote-hg` helper)
 
+'GIT_MERGE_ALLOW_UNRELATED_HISTORIES'::
+	Allow "git merge" to merge unrelated histories by default.
+
 
 Discussion[[Discussion]]
 ------------------------
diff --git a/builtin/merge.c b/builtin/merge.c
index e3db41b..4e8b1a1 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -1191,6 +1191,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		head_commit = lookup_commit_or_die(head_sha1, "HEAD");
 
 	git_config(git_merge_config, NULL);
+	if (getenv("GIT_MERGE_ALLOW_UNRELATED_HISTORIES"))
+		allow_unrelated_histories =
+			git_env_bool("GIT_MERGE_ALLOW_UNRELATED_HISTORIES", 0);
 
 	if (branch_mergeoptions)
 		parse_branch_merge_options(branch_mergeoptions);
diff --git a/builtin/pull.c b/builtin/pull.c
index 797932d..4e886a5 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -86,7 +86,7 @@ static char *opt_verify_signatures;
 static struct argv_array opt_strategies = ARGV_ARRAY_INIT;
 static struct argv_array opt_strategy_opts = ARGV_ARRAY_INIT;
 static char *opt_gpg_sign;
-static int opt_allow_unrelated_histories;
+static int opt_allow_unrelated_histories = -1; /* unspecified */
 
 /* Options passed to git-fetch */
 static char *opt_all;
@@ -159,6 +159,9 @@ static struct option pull_options[] = {
 	OPT_SET_INT(0, "allow-unrelated-histories",
 		    &opt_allow_unrelated_histories,
 		    N_("allow merging unrelated histories"), 1),
+	OPT_SET_INT(0, "no-allow-unrelated-histories",
+		    &opt_allow_unrelated_histories,
+		    N_("do not allow merging unrelated histories"), 0),
 
 	/* Options passed to git-fetch */
 	OPT_GROUP(N_("Options related to fetching")),
@@ -609,6 +612,8 @@ static int run_merge(void)
 		argv_array_push(&args, opt_gpg_sign);
 	if (opt_allow_unrelated_histories > 0)
 		argv_array_push(&args, "--allow-unrelated-histories");
+	else if (!opt_allow_unrelated_histories)
+		argv_array_push(&args, "--no-allow-unrelated-histories");
 
 	argv_array_push(&args, "FETCH_HEAD");
 	ret = run_command_v_opt(args.argv, RUN_GIT_CMD);
-- 
2.8.1-422-g6d9b748