Remove "Commit:" field from --version output

If nvim is built from a non-tagged commit, the truncated commit hash is
already appended to the main version string (e.g., "NVIM v0.1.0-83-g959f260 ..."),
making the "Commit:" field redundant.

Regarding the truncated hash length: we don't have nearly enough commits
to worry about collisions, and probably won't ever, so the default
length should be fine.

src/nvim/version.c

@@ -37,9 +37,6 @@ char *Version = VIM_VERSION_SHORT;
 char *longVersion = NVIM_VERSION_LONG;
 char *longVersionWithDate = NVIM_VERSION_LONG \
                             " (compiled " __DATE__ " " __TIME__ ")";
-#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;
 
@@ -1099,9 +1096,6 @@ 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);