cmdutil: make commit message shown in text editor customizable by template This patch makes commit message shown in text editor customizable by template. For example, this can advertise: - sample commit messages for routine works, - points to call attention before commit, - message of the day, and so on To show commit message correctly even in problematic encoding, this patch chooses the latter below: - replace "buildcommittext" with "buildcommittemplate" completely - invoke "buildcommittemplate" only if '[committemplate] changeset' is configured explicitly For example, if multibyte character ending with backslash (0x5c) is followed by ASCII character 'n' in the customized template, sequence of backslash and 'n' is treated as line-feed unexpectedly (and multibyte character is broken, too). This corruption occurs in 'decode("string-escape")' while parsing template string.