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>

editor.h

@@ -0,0 +1,34 @@
+#ifndef EDITOR_H
+#define EDITOR_H
+
+struct strbuf;
+
+const char *git_editor(void);
+const char *git_sequence_editor(void);
+int is_terminal_dumb(void);
+
+/**
+ * Launch the user preferred editor to edit a file and fill the buffer
+ * with the file's contents upon the user completing their editing. The
+ * third argument can be used to set the environment which the editor is
+ * run in. If the buffer is NULL the editor is launched as usual but the
+ * file's contents are not read into the buffer upon completion.
+ */
+int launch_editor(const char *path, struct strbuf *buffer,
+		  const char *const *env);
+
+int launch_sequence_editor(const char *path, struct strbuf *buffer,
+			   const char *const *env);
+
+/*
+ * In contrast to `launch_editor()`, this function writes out the contents
+ * of the specified file first, then clears the `buffer`, then launches
+ * the editor and reads back in the file contents into the `buffer`.
+ * Finally, it deletes the temporary file.
+ *
+ * If `path` is relative, it refers to a file in the `.git` directory.
+ */
+int strbuf_edit_interactively(struct strbuf *buffer, const char *path,
+			      const char *const *env);
+
+#endif