editor: move editor-related functions and declarations into common file

cache.h and strbuf.[ch] had editor-related functions.  Move these into
editor.[ch].

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/editor.c b/editor.c
index 3bea3ef..12025df 100644
--- a/editor.c
+++ b/editor.c
@@ -2,12 +2,14 @@
 #include "abspath.h"
 #include "advice.h"
 #include "config.h"
+#include "editor.h"
 #include "environment.h"
 #include "gettext.h"
 #include "strbuf.h"
 #include "strvec.h"
 #include "run-command.h"
 #include "sigchain.h"
+#include "wrapper.h"
 
 #ifndef DEFAULT_EDITOR
 #define DEFAULT_EDITOR "vi"
@@ -130,3 +132,31 @@ int launch_sequence_editor(const char *path, struct strbuf *buffer,
 {
 	return launch_specified_editor(git_sequence_editor(), path, buffer, env);
 }
+
+int strbuf_edit_interactively(struct strbuf *buffer, const char *path,
+			      const char *const *env)
+{
+	char *path2 = NULL;
+	int fd, res = 0;
+
+	if (!is_absolute_path(path))
+		path = path2 = xstrdup(git_path("%s", path));
+
+	fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
+	if (fd < 0)
+		res = error_errno(_("could not open '%s' for writing"), path);
+	else if (write_in_full(fd, buffer->buf, buffer->len) < 0) {
+		res = error_errno(_("could not write to '%s'"), path);
+		close(fd);
+	} else if (close(fd) < 0)
+		res = error_errno(_("could not close '%s'"), path);
+	else {
+		strbuf_reset(buffer);
+		if (launch_editor(path, buffer, env) < 0)
+			res = error_errno(_("could not edit '%s'"), path);
+		unlink(path);
+	}
+
+	free(path2);
+	return res;
+}