Stop trying to run more actions when one fails

Although many failures do simply die, there are some modes in which we
can fail to stop if an error occurs.  Fix these by sprinkling "|| exit"
judiciously in the command handling.

Signed-off-by: John Keeping <john@keeping.me.uk>
Fix error reporting if "git --exec-path" fails

Because we have not found and sourced git-sh-setup, we can't use "die"
here, so replace it with echo and exit.

Signed-off-by: John Keeping <john@keeping.me.uk>
Check that git-stripspace supports "--comment-lines"

It seems that versions of Git older than 1.8.2 are still in use in the
wild, so check that the installed version of Git supports the features
we need.  This avoids exposing the user to a confusing error message
when the Git version is too old.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'stop-if-action-fails' into pu

* branch stop-if-action-fails:
  Stop trying to run more actions when one fails

Fix handling of certain error scenarios.
Merge branch 'fix-git-detect-error' into pu

* branch fix-git-detect-error:
  Fix error reporting if "git --exec-path" fails

Don't use an undefined function if finding Git fails.
Merge branch 'check-git-version' into pu

* branch check-git-version:
  Check that git-stripspace supports "--comment-lines"

Avoid displaying a cryptic error message when the installed Git
version is too old and so does not support the --comment-lines option
to git-stripspace.

This may be better implemented as a sanity check in the makefile.