Fix/revisit git-describe enhancement (#11124)

* Fix/keep massaging git-describe result

Ref: https://github.com/neovim/neovim/pull/11117#issuecomment-536416223

* build: revisit generation of version from Git

Fixes "make clean && make", where "auto/versiondef.h" would be missing
since b18b84d - because BYPRODUCTS are apparently removed when cleaning.

This includes the following improvements/changes:

- do not run git-describe during CMake's configure phase just for
  reporting
- do not print with changed Git version (too noisy, simplifies code)

* Move to src/nvim (included before config) for easier flow

* fallback to describe always, write empty include file

* update_version_stamp.lua: use prefix always

scripts/update_version_stamp.lua

@@ -4,11 +4,11 @@
 -- This is called via the custom update_version_stamp target in
 -- src/nvim/CMakeLists.txt.
 --
--- arg[1]: file containing the last git-describe output
--- arg[2]: file in which to update the version string
+-- arg[1]: file in which to update the version string
+-- arg[2]: prefix to use always ("vX.Y.Z")
 
 local function die(msg)
-  print(string.format('%s: %s', arg[0], msg))
+  io.stderr:write(string.format('%s: %s\n', arg[0], msg))
   -- No error, fall back to using generated "-dev" version.
   os.exit(0)
 end
@@ -17,29 +17,34 @@ if #arg ~= 2 then
   die(string.format("Expected two args, got %d", #arg))
 end
 
-local stampfile = arg[1]
-local stamp = io.open(stampfile, 'r')
-if stamp then
-  stamp = stamp:read('*l')
+local versiondeffile = arg[1]
+local prefix = arg[2]
+
+local described = io.popen('git describe --dirty'):read('*l')
+if not described then
+  described = io.popen('git describe --tags --always --dirty'):read('*l')
+end
+if not described then
+  io.open(versiondeffile, 'w'):write('\n')
+  die('git-describe failed, using empty include file.')
 end
 
-local current = io.popen('git describe --dirty'):read('*l')
-if not current then
-  die('git-describe failed')
+-- `git describe` annotates the most recent tagged release; for pre-release
+-- builds we must replace that with the unreleased version.
+local with_prefix = described:gsub("^v%d+%.%d+%.%d+", prefix)
+if described == with_prefix then
+  -- Prepend the prefix always, e.g. with "nightly-12208-g4041b62b9".
+  with_prefix = prefix .. "-" .. described
 end
 
-if stamp ~= current then
-  if stamp then
-    print(string.format('git version changed: %s -> %s', stamp, current))
-  end
-  local new_lines = {}
-  local versiondeffile = arg[2]
-  for line in io.lines(versiondeffile) do
-    if line:match("NVIM_VERSION_MEDIUM") then
-      line = '#define NVIM_VERSION_MEDIUM "'..current..'"'
-    end
-    new_lines[#new_lines + 1] = line
-  end
-  io.open(versiondeffile, 'w'):write(table.concat(new_lines, '\n') .. '\n')
-  io.open(stampfile, 'w'):write(current)
+-- Read existing include file.
+local current = io.open(versiondeffile, 'r')
+if current then
+  current = current:read('*l')
+end
+
+-- Write new include file, if different.
+local new = '#define NVIM_VERSION_MEDIUM "'..with_prefix..'"'
+if current ~= new then
+  io.open(versiondeffile, 'w'):write(new .. '\n')
 end