repository: add a getter and remove function for git's prepared message

The 'git revert/cherry-pick/merge -n' commands leave .git/MERGE_MSG
behind so that git-commit can find it. As we don't yet support these
operations, users who are shelling out to let git perform these
operations haven't had a convenient way to get this message.

These functions allow the user to retrieve the message and remove it
when she's created the commit.

include/git2/repository.h

@@ -315,6 +315,28 @@ GIT_EXTERN(int) git_repository_index(git_index **out, git_repository *repo);
  */
 GIT_EXTERN(void) git_repository_set_index(git_repository *repo, git_index *index);
 
+/**
+ * Retrive git's prepared message
+ *
+ * Operations such as git revert/cherry-pick/merge with the -n option
+ * stop just short of creating a commit with the changes and save
+ * their prepared message in .git/MERGE_MSG so the next git-commit
+ * execution can present it to the user for them to amend if they
+ * wish.
+ *
+ * Use this function to get the contents of this file. Don't forget to
+ * remove the file after you create the commit.
+ */
+GIT_EXTERN(int) git_repository_message(char *buffer, size_t len, git_repository *repo);
+
+/**
+ * Remove git's prepared message.
+ *
+ * Remove the message that `git_repository_message` retrieves.
+ */
+GIT_EXTERN(int) git_repository_message_remove(git_repository *repo);
+
+
 /** @} */
 GIT_END_DECL
 #endif

src/repository.c

@@ -1071,3 +1071,59 @@ int git_repository_head_tree(git_tree **tree, git_repository *repo)
 	*tree = (git_tree *)obj;
 	return 0;
 }
+
+#define MERGE_MSG_FILE "MERGE_MSG"
+
+int git_repository_message(char *buffer, size_t len, git_repository *repo)
+{
+        git_buf buf = GIT_BUF_INIT, path = GIT_BUF_INIT;
+        struct stat st;
+        ssize_t size;
+        int error;
+
+        if (git_buf_joinpath(&path, repo->path_repository, MERGE_MSG_FILE) < 0)
+                return -1;
+
+        error = p_stat(git_buf_cstr(&path), &st);
+        if (error < 0) {
+		if (errno == ENOENT)
+			error = GIT_ENOTFOUND;
+
+                git_buf_free(&path);
+                return error;
+        }
+
+        if (buffer == NULL) {
+                git_buf_free(&path);
+                return st.st_size;
+        }
+
+        if (git_futils_readbuffer(&buf, git_buf_cstr(&path)) < 0)
+                goto on_error;
+
+        memcpy(buffer, git_buf_cstr(&buf), len);
+        size = git_buf_len(&buf);
+
+        git_buf_free(&path);
+        git_buf_free(&buf);
+        return size;
+
+on_error:
+        git_buf_free(&path);
+        return -1;
+
+}
+
+int git_repository_message_remove(git_repository *repo)
+{
+        git_buf path = GIT_BUF_INIT;
+        int error;
+
+        if (git_buf_joinpath(&path, repo->path_repository, MERGE_MSG_FILE) < 0)
+                return -1;
+
+	error = p_unlink(git_buf_cstr(&path));
+	git_buf_free(&path);
+
+	return error;
+}

tests-clar/repo/message.c

@@ -0,0 +1,47 @@
+#include "clar_libgit2.h"
+#include "buffer.h"
+#include "refs.h"
+#include "posix.h"
+
+static git_repository *_repo;
+static git_buf _path;
+static char *_actual;
+
+void test_repo_message__initialize(void)
+{
+        _repo = cl_git_sandbox_init("testrepo.git");
+}
+
+void test_repo_message__cleanup(void)
+{
+        cl_git_sandbox_cleanup();
+	git_buf_free(&_path);
+	git__free(_actual);
+	_actual = NULL;
+}
+
+void test_repo_message__none(void)
+{
+	cl_assert_equal_i(GIT_ENOTFOUND, git_repository_message(NULL, 0, _repo));
+}
+
+void test_repo_message__message(void)
+{
+	const char expected[] = "Test\n\nThis is a test of the emergency broadcast system\n";
+	ssize_t len;
+
+	cl_git_pass(git_buf_joinpath(&_path, git_repository_path(_repo), "MERGE_MSG"));
+        cl_git_mkfile(git_buf_cstr(&_path), expected);
+
+        len = git_repository_message(NULL, 0, _repo);
+        cl_assert(len > 0);
+        _actual = git__malloc(len + 1);
+        cl_assert(_actual != NULL);
+
+        cl_assert(git_repository_message(_actual, len, _repo) > 0);
+        _actual[len] = '\0';
+        cl_assert_equal_s(expected, _actual);
+
+	cl_git_pass(p_unlink(git_buf_cstr(&_path)));
+	cl_assert_equal_i(GIT_ENOTFOUND, git_repository_message(NULL, 0, _repo));
+}