[RFC/PATCH 1/2] Introduce CHERRY_HEAD

[Jay Soffian <jaysoffian@gmail.com>]

When a cherry-pick conflicts git advises to use:

 $ git commit -c <original commit id>

to preserve the original commit message and authorship. Instead,
let's record the original commit id in CHERRY_HEAD and advise to use:

  $ git commit -c CHERRY_HEAD

In the next commit, we teach git to handle the '-c CHERRY_HEAD' part.
---
 branch.c         |    1 +
 builtin/commit.c |    1 +
 builtin/revert.c |   23 ++++++++++++++++++++---
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/branch.c b/branch.c
index 93dc866..81bbf95 100644
--- a/branch.c
+++ b/branch.c
@@ -217,6 +217,7 @@ void create_branch(const char *head,
 
 void remove_branch_state(void)
 {
+	unlink(git_path("CHERRY_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));
diff --git a/builtin/commit.c b/builtin/commit.c
index 03cff5a..8850621 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1424,6 +1424,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		die("cannot update HEAD ref");
 	}
 
+	unlink(git_path("CHERRY_HEAD"));
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("MERGE_MODE"));
diff --git a/builtin/revert.c b/builtin/revert.c
index dc1b702..c373e69 100644
--- a/builtin/revert.c
+++ b/builtin/revert.c
@@ -248,6 +248,22 @@ static void set_author_ident_env(const char *message)
 			sha1_to_hex(commit->object.sha1));
 }
 
+static void write_cherry_head(void)
+{
+	int fd;
+	struct strbuf buf = STRBUF_INIT;
+
+	strbuf_addf(&buf, "%s\n", sha1_to_hex(commit->object.sha1));
+
+	fd = open(git_path("CHERRY_HEAD"), O_WRONLY | O_CREAT, 0666);
+	if (fd < 0)
+		die_errno("Could not open '%s' for writing",
+			  git_path("CHERRY_HEAD"));
+	if (write_in_full(fd, buf.buf, buf.len) != buf.len)
+		die_errno("Could not write to '%s'", git_path("CHERRY_HEAD"));
+	close(fd);
+}
+
 static void advise(const char *advice, ...)
 {
 	va_list params;
@@ -269,9 +285,10 @@ static void print_advice(void)
 	advise("after resolving the conflicts, mark the corrected paths");
 	advise("with 'git add <paths>' or 'git rm <paths>'");
 
-	if (action == CHERRY_PICK)
-		advise("and commit the result with 'git commit -c %s'",
-		       find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
+	if (action == CHERRY_PICK) {
+		write_cherry_head();
+		advise("and commit the result with 'git commit -c CHERRY_HEAD'");
+	}
 }
 
 static void write_message(struct strbuf *msgbuf, const char *filename)
-- 
1.7.4.5.g9affb