Update distclean rule to preserve tarball contents

[koutcher]

... and replace it with new realclean rule.

Closes #1167

[krobelus]

cc @sbakker

I also thought that make clean deletes all generated files, while make distclean preserves things like documentation and configure scripts.

In Tig it's the other way round, which is weird. However, Git does the same, so we should first understand why to see if it makes sense for us to deviate.

AFACT this behavior was originally introduced to Git in git/git@922aa80
this is the log message:

Do not remove distributed configure script

Before this patch the clean target has removed the
configure script that comes with Git tar file.
That made compiling Git for different architectures
inconvenient.
This patch excludes configure from the files to be
deleted by 'make clean' and adds new target 'distclean'
to preserve old functionality.

(if we stick to a separate target, maybe clean-all is a more familiar/discoverable name)

[sbakker]

I'm not stuck on the pristine name, it's just something I saw occasionally (possibly along with mrproper in the Linux kernel source). The Standard Targets section in the GNU make manual defines clean and distclean:

• clean - delete files created by the Makefile during the build (compilation) process (.o, .a, .so, etc.), but not configuration files.
• distclean - delete all files that are created by configuring or building the program (so also config.status, but not the configure script). It should return the state of the directory to that of the unpacked tarball.

There's also a maintainer-clean, but even that one is not supposed to delete the configure script. So veryclean is probably the best choice here.

[koutcher]

veryclean would be good for me.

[koutcher]

What makes things a bit more complicated with Tig is the two-stage delivery process which adds generated artefacts that may not be reproducible to the release branch before using it to build the tarball. The current distclean behaviour is consistent from the point view of the master branch but not when you get a tarball. As for the name, I didn't find enough reference to pristine proposed by @sbakker, so I preferred realclean which is mentioned in Make documentation, though often synonym to distclean. With clean-all there is a risk of confusion with the classic make clean all.