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.

DEVEL/Developer.txt

@@ -92,10 +92,11 @@ B. Specify the prefix for variable substitution:
 	tree you cloned from git.  I use ~/nethack/GITADDDIR; for that base,
 	create the needed directories and edit the file:
 		~/nethack/GITADDDIR/DOTGIT/PRE
-	Put this in it (if your OS is not Unix-like you may need to change
-	the first line):
+	Put this in it, adapting it to your variant (if your OS is not Unix-like
+	you may need to change the first line):
 		#!/bin/sh
 		git config nethack.substprefix MINE
+		git config nethack.projectname MineHack
 	Now make it executable:
 		chmod +x ~/nethack/GITADDDIR/DOTGIT/PRE
 C. Configure the repository:
@@ -169,8 +170,8 @@ A. Introduction
    The PREFIX is the value in the git config variable nethack.substprefix.
    VARNAME is one of:
 	Date
-	Branch (experimental)
-	Revision (experimental)
+	Branch
+	Revision
    other names will give a warning.
 
 B. Enabling variable expansion