git-gui: prefer shell at "/bin/sh" with Meson

Meson detects the path of the target shell via `find_program("sh")`,
which essentially does a lookup via `PATH`. We know that almost all
systems have "/bin/sh" available though, which makes it the superior
choice as a default value.

Adapt `find_program()` to prefer "/bin/sh" over any other "sh"
executable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>

meson.build

@@ -4,7 +4,7 @@ project('git-gui',
 
 fs = import('fs')
 
-shell = find_program('sh')
+shell = find_program('/bin/sh', 'sh')
 tclsh = find_program('tclsh')
 wish = find_program('wish')