Fix build_version.cc generation error if GIT not found (#7916)

Summary:
(Fixes a regression introduced in the build_version generation PR https://github.com/facebook/rocksdb/issues/7866 )

In the Makefile case, needed to ignore stderr on the tag (everywhere else was fine).

In the CMAKE case, no GIT implies "changes" so that we use the system date rather than the empty GIT date.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7916

Test Plan: Built in a tree that did not contain the ".git" directory.  Validated that no errors appeared during the build process and that the build version date was not empty.

Reviewed By: jay-zhuang

Differential Revision: D26169203

Pulled By: mrambacher

fbshipit-source-id: 3288a23b48d97efed5e5b38c9aefb3ef1153fa16

CMakeLists.txt

@@ -185,7 +185,7 @@ if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
   endif()
 else()
   set(GIT_SHA 0)
-  set(GIT_MOD 0)
+  set(GIT_MOD 1)
 endif()
 string(REGEX REPLACE "[^0-9a-fA-F]+" "" GIT_SHA "${GIT_SHA}")
 string(REGEX REPLACE "[^0-9: /-]+" "" GIT_DATE "${GIT_DATE}")

Makefile

@@ -853,7 +853,7 @@ ifdef FORCE_GIT_SHA
 	git_date := $(build_date)
 else
 	git_sha := $(shell git rev-parse HEAD 2>/dev/null)
-	git_tag  := $(shell git symbolic-ref -q --short HEAD || git describe --tags --exact-match 2>/dev/null)
+	git_tag  := $(shell git symbolic-ref -q --short HEAD 2> /dev/null || git describe --tags --exact-match 2>/dev/null)
 	git_mod  := $(shell git diff-index HEAD --quiet 2>/dev/null; echo $$?)
 	git_date := $(shell git log -1 --date=format:"%Y-%m-%d %T" --format="%ad" 2>/dev/null)
 endif