nhgitset version 4

To update, run "perl DEVEL/nhgitset.pl"

Fixes:
- "nhcommit -a" has been fixed
- NHDT was hardwired in places
- no longer complain about a missing dat directory outside of the
    NetHack source tree
- make update of gitinfo atomic
- Replace some hardwired directory separators with OS-dependent constructs

Backwards Incompatibilities:
- NH_DATESUB's DATE() is now Date() to match the other variables
- MSYS2 requires an additional Perl package - the MSYS2 docs have
    been updated

New Help System:
- git nhhelp
   This command mirrors "git help" for nh* commands.
- See git nhhelp nhsub for general help on substitution variables

New Substitution Variables:
-Brev()
    An aBREViation of $PREFIX-Branch$:$PREFIX-Revision$ - this
    may help get line length under control in file headers.
-Assert(TYPE=VALUE)
    If TYPE does not match VALUE, do not substitute on this line.
    TYPE P checks VALUE against nethack.substprefix
-Project(arg)
    Returns nethack.projectname if there is no arg and an uppercase
    version if arg is uc.

Other New Features:
- Add nethack.projectname
- Documentation updates - see "git nhhelp nhsub"
- On checkout or merge of a branch, check for nhgitset version updates
  and provide an optional message to the user.
- Move NH_DATESUB substitutions here from cron job to keep dates in sync
- PREFIX-* keywords now available in NH_DATESUB templates
- Support use of nhgitset.pl from a different repo; note that update
  checks will be dependent on keeping the original source repo up-to-date
  and in the same location.

doc/nethack.6

@@ -1,4 +1,4 @@
-.\"DO NOT REMOVE NH_DATESUB .TH NETHACK 6 "DATE(%-d %B %Y)" NETHACK
+.\"DO NOT REMOVE NH_DATESUB .TH NETHACK 6 "DATE(%-d %B %Y)" Project(uc)
 .TH NETHACK 6 "21 February 2022" NETHACK
 .\"DO NOT REMOVE NH_DATESUB .ds Nd DATE(%Y)
 .ds Nd 2022