Remove erroneously-placed backticks when describing a lack of GNU coreutils.

Don't rely on $OSTYPE being available, which it isn't in strict POSIX mode. Ubuntu's sh is often alised to dash, and hence this fails. Instead, default to using the output from 'uname'.

Add Ubuntu as a dedicated CI environment. By default, sh is aliase to dash which often provides interesting challenges for portability, so making this work here is a good thing as mistakes will be spotted earlier.

When running tests on non-Linux systems, it was just assumed that GNU coreutils had been installed. This meant that the wrapper commands for date/ln on those systems always had gdate and gln installed. Rather than assume that, check to see if the GNU coreutils versions as present, and if not echo a warning.

PLATFORM is populated when running via `./configure && make` but this isn't guaranteed if an individual test is run directly, such as `regress/cmdline/tag.sh`. In such cases, PLATFORM will be empty, but we still want to use it. Since we test for non-linux values, only set PLATFORM if we're running on Linu so that the correct commands are used.

Remove comments which are no longer required.

Increase/decrease the currently active split. New dimensions persist for the tog session, irrespective of subsequent closures and new child views. suggested by and ok stsp@

If in a vertical (horizontal) split, switch to horizontal (vertical), and make new split mode persist. ok op@

ok stsp@

got_privsep_recv_imsg users don't need to bother to look whether the imsg type is GOT_IMSG_ERROR because got_privsep_recv_imsg already takes care of that and turns GOT_IMSG_ERROR into `struct got_error's automatically. ok stsp@

ok stsp@

Remove those includes which are searched for portably.

Left-over from how OpenRCS/OpenCVS do error handling. OK stsp@

GNU dd only accepts upper-case letters for the size units.

Setting this option will cause 'got tag' to sign all created tags using the SSH key, unless overridden by the -s flag. ok stsp@

Don't rely on '-r' for date as it's not consistent across linuxses. Instead, expand the variables required and use '-d'. OK thomas@

Add missing files for testing fetch.

When cherry-picking commits from main, do so in reverse order so they match more linearly with main.