Log Message:
mikutter: update to 3.9.6.

Upstream changes:

mikutter 3.9.6

* [appimage] update ruby to 2.6.5
* [appimage] update jemalloc to 5.2.1
* change notation of credits in "about mikutter" in the settings
 * change author's names to Mastodon accts registered at Redmine
 * add yuntan_t to the authors
* support a new illustration URL format of pixiv
 * thanks Akira Ouchi
* extract tabs cannot be removed
 * thanks Shibafu Midorino
* crash when scale value is not returned properly
 * thanks ahiru ahiru
* several files opened read mode were overwriten by FileUtils.mv
  on migrating from 3.6 and prior
 * this might be problematic on Windows environment and
   will no longer appear as an actual problem, but anyway fixed.
* show search tab if at least one search spell is defined
 * thanks Akira Ouchi
 * turns out this merge was missed for a year