version: Prepare for releases.

* Hide commit information from --version if we can't find any (e.g. when
  building from tarball).

To define a release in CMake, set NVIM_VERSION_PRERELEASE to "".
This will modify --version output to:

    * Show annotated Git tag instead of commit hash (NVIM_VERSION_COMMIT).
    * Hide commit date (NVIM_VERSION_BUILD).

src/nvim/version.c

@@ -36,7 +36,9 @@ char *Version = VIM_VERSION_SHORT;
 char *longVersion = NVIM_VERSION_LONG;
 char *longVersionWithDate = NVIM_VERSION_LONG " (compiled " __DATE__ " " __TIME__ ")";
 char *mediumVersion = NVIM_VERSION_MEDIUM;
+#ifdef NVIM_VERSION_COMMIT
 char *version_commit = "Commit: " NVIM_VERSION_COMMIT;
+#endif
 char *version_buildtype = "Build type: " NVIM_VERSION_BUILD_TYPE;
 char *version_cflags = "Compilation: " NVIM_VERSION_CFLAGS;
 
@@ -1024,7 +1026,9 @@ void list_version(void)
   // When adding features here, don't forget to update the list of
   // internal variables in eval.c!
   MSG(longVersionWithDate);
+#ifdef NVIM_VERSION_COMMIT
   MSG(version_commit);
+#endif
   MSG(version_buildtype);
   MSG(version_cflags);