version: generate "build number" from commit timestamp

- cmake: git_timestamp() returns last commit time formatted as
  `YYYYMMddHHmm`.
- Always include commit hash in :version and --version output.

`nvim --version` sample output:
  NVIM 0.0.0-alpha+201410070245 (compiled Oct  7 2014 05:30:45)
  Commit: f747b2b1ff7bfe7eb00cc2be82d7af87c98f1111

src/nvim/version.c

@@ -18,9 +18,8 @@
 #include "nvim/version_defs.h"
 
 char *Version = VIM_VERSION_SHORT;
-
-char *longVersion = NVIM_VERSION_LONG_DATE __DATE__ " " __TIME__ ")";
-
+char *longVersion = NVIM_VERSION_LONG " (compiled " __DATE__ " " __TIME__ ")";
+char *version_commit = "Commit: " NVIM_VERSION_COMMIT;
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "version.c.generated.h"
@@ -747,6 +746,7 @@ void list_version(void)
   // When adding features here, don't forget to update the list of
   // internal variables in eval.c!
   MSG(longVersion);
+  MSG(version_commit);
 
   // Print the list of extra patch descriptions if there is at least one.
   char *s = "";