git-gui: make _shellpath usable on startup

git-gui: make _shellpath usable on startup

Since commit d5257fb3c1de (git-gui: handle textconv filter on
Windows and in development, 2010-08-07), git-gui will search for a
usable shell if _shellpath is not configured, and on Windows may
resort to using auto_execok to find 'sh'. While this was intended for
development use, checks are insufficient to assure a proper
configuration when deployed where _shellpath is always set, but might
not give a usable shell.

Let's make this more robust by only searching if _shellpath was not
defined, and then using only our restricted search functions.
Furthermore, we should convert to a Windows path on Windows.  Always
check for a valid shell on startup, meaning an absolute path to an
executable, aborting if these conditions are not met.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>