build: gracefully handle error in git-version #19289

- only update git-version if both of these conditions are met:
    - `git` command succeeds
    - `versiondef_git.h` would change (SHA1-diff)
- else print a status/warning message

also move version generation out of Lua into cmake.

cmake/GenerateVersion.cmake

@@ -0,0 +1,47 @@
+# Handle generating version from Git.
+set(use_git_version 0)
+if(NVIM_VERSION_MEDIUM)
+  message(STATUS "USING NVIM_VERSION_MEDIUM: ${NVIM_VERSION_MEDIUM}")
+  return()
+endif()
+
+find_program(GIT_EXECUTABLE git)
+if(NOT GIT_EXECUTABLE)
+  message(AUTHOR_WARNING "Skipping version-string generation (cannot find git)")
+  return()
+endif()
+
+execute_process(
+  COMMAND git describe --first-parent --tags --always --dirty
+  OUTPUT_VARIABLE GIT_TAG
+  ERROR_VARIABLE ERR
+  RESULT_VARIABLE RES
+)
+
+if("${RES}" EQUAL 1)
+  if(EXISTS ${OUTPUT})
+    message(STATUS "Unable to extract version-string from git: keeping the last known version")
+  else()
+    # this will only be executed once since the file will get generated afterwards
+    message(AUTHOR_WARNING "Git tag extraction failed with: " "${ERR}")
+    file(WRITE "${OUTPUT}" "")
+  endif()
+  return()
+endif()
+
+string(STRIP "${GIT_TAG}" GIT_TAG)
+string(REGEX REPLACE "^v[0-9]+.[0-9]+.[0-9]+-" "" NVIM_VERSION_GIT "${GIT_TAG}")
+set(NVIM_VERSION_MEDIUM
+    "v${NVIM_VERSION_MAJOR}.${NVIM_VERSION_MINOR}.${NVIM_VERSION_PATCH}-dev-${NVIM_VERSION_GIT}"
+)
+set(NVIM_VERSION_STRING "#define NVIM_VERSION_MEDIUM \"${NVIM_VERSION_MEDIUM}\"\n")
+string(SHA1 CURRENT_VERSION_HASH "${NVIM_VERSION_STRING}")
+
+if(EXISTS ${OUTPUT})
+  file(SHA1 "${OUTPUT}" NVIM_VERSION_HASH)
+endif()
+
+if(NOT "${NVIM_VERSION_HASH}" STREQUAL "${CURRENT_VERSION_HASH}")
+  message(STATUS "Updating NVIM_VERSION_MEDIUM: ${NVIM_VERSION_MEDIUM}")
+  file(WRITE "${OUTPUT}" "${NVIM_VERSION_STRING}")
+endif()