git doesn't support "-C" args under 1.8.5 occurs in configure.ac

as I noticed from git release notes in 1.8.5: 

 * Just like "make -C <directory>", "git -C <directory> ..." tells Git
   to go there before doing anything else.

So we should have some conditional statements in configure.ac around git checkpoint, otherwise we got error message which is :

Unknown option: -C
usage: git [--version] [--help] [-c name=value]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p|--paginate|--no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           <command> [<args>]

Encounter the same problem on centos7 and git version is 1.8.3. Instead of checking the version to use -C option, maybe we could just use --git-dir for the same purpose?

--- a/configure
+++ b/configure
@@ -2749,9 +2749,9 @@ HAS_GIT=no-repository
 fi
 if test $HAS_GIT = found
 then
-    GITVERSION="git -C \$(srcdir) rev-parse --short HEAD"
-    GITTAG="git -C \$(srcdir) describe --all --always --dirty"
-    GITBRANCH="git -C \$(srcdir) name-rev --name-only HEAD"
+    GITVERSION="git --git-dir \$(srcdir)/.git rev-parse --short HEAD"
+    GITTAG="git --git-dir \$(srcdir)/.git describe --all --always --dirty"
+    GITBRANCH="git --git-dir \$(srcdir)/.git name-rev --name-only HEAD"
 else
     GITVERSION=""
     GITTAG=""
diff --git a/configure.ac b/configure.ac
index b13728e..278ed9f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -39,9 +39,9 @@ HAS_GIT=no-repository
 fi
 if test $HAS_GIT = found
 then
-    GITVERSION="git -C \$(srcdir) rev-parse --short HEAD"
-    GITTAG="git -C \$(srcdir) describe --all --always --dirty"
-    GITBRANCH="git -C \$(srcdir) name-rev --name-only HEAD"
+    GITVERSION="git --git-dir \$(srcdir)/.git rev-parse --short HEAD"
+    GITTAG="git --git-dir \$(srcdir)/.git describe --all --always --dirty"
+    GITBRANCH="git --git-dir \$(srcdir)/.git name-rev --name-only HEAD"
 else
     GITVERSION=""
     GITTAG=""

This could give me the right sys._git info.

Thanks for the PR.  I agree with Xiang Zhang that using --git-dir appears to be a better solution.  See other review comments on the PR.

New changeset 4c8555773a801f957297132a92c0acb382d640e4 by Xiang Zhang in branch 'master':
bpo-30411: Use --git-dir instead of -C to make git work under version below 1.8.5. (GH-8744)
https://github.com/python/cpython/commit/4c8555773a801f957297132a92c0acb382d640e4

New changeset 2011dd77b6bd6f02f54ceef9a80a22bd373b66e2 by Miss Islington (bot) in branch '3.7':
bpo-30411: Use --git-dir instead of -C to make git work under version below 1.8.5. (GH-8744)
https://github.com/python/cpython/commit/2011dd77b6bd6f02f54ceef9a80a22bd373b66e2

New changeset 95f9e14e33b713f35f5368acf7be56750bbdb5a8 by Xiang Zhang in branch '2.7':
[2.7] bpo-30411: Use --git-dir instead of -C to make git work under version below 1.8.5. (GH-8744) (GH-8838)
https://github.com/python/cpython/commit/95f9e14e33b713f35f5368acf7be56750bbdb5a8