git-gui: Add support of SHA256 repo

git-gui: Add support of SHA256 repo

This patch adds the basic support of SHA256 Git repositories.
Most of changes are idiomatic replacement of the hard-coded hash ID
length, but there are subtle things:

* The hash length is determined on startup, and stored in $hashlength
  global variable (either 40 or 64).
* The hard-coded "40" are replaced with $hashlength;
  for regexp patterns, the ugly string map is used.
* Some code have the fixed numbers like 39 and 45, and those are
  replaced with the $hashlength and the offset correction.
* $nullid and $nullid2 are generated for the hash length.

A caveat is that repository picker dialog is performed before
evaluating the repo type, hence $hashlength isn't set there yet.
So the code dealing with the hard-coded "40" are handled differently;
namely, the regexp range is expanded, and the null id is generated
from the HEAD id length locally.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>