bpo-27593: Get SCM build info from git instead of hg (#1327)

Based on commit 5c4b0d063aba0a68c325073f5f312a2c9f40d178 by Ned
Deily, which is based on original patches by Brett Cannon and Steve
Dower.

Remove also the private _Py_svnversion() function and SVNVERSION
variable.

Note: Py_SubversionRevision() and Py_SubversionShortBranch() are
unchanged, they are part of the public API.
diff --git a/configure b/configure
index 35fbeac..7d1f4ad 100755
--- a/configure
+++ b/configure
@@ -683,11 +683,10 @@
 INSTALL_DATA
 INSTALL_SCRIPT
 INSTALL_PROGRAM
-HAS_HG
-HGBRANCH
-HGTAG
-HGVERSION
-SVNVERSION
+HAS_GIT
+GITBRANCH
+GITTAG
+GITVERSION
 ARFLAGS
 ac_ct_AR
 AR
@@ -5689,15 +5688,20 @@
 fi
 
 
-# Extract the first word of "svnversion", so it can be a program name with args.
-set dummy svnversion; ac_word=$2
+
+
+
+if test -e $srcdir/.git/HEAD
+then
+# Extract the first word of "git", so it can be a program name with args.
+set dummy git; ac_word=$2
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
 $as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_prog_SVNVERSION+:} false; then :
+if ${ac_cv_prog_HAS_GIT+:} false; then :
   $as_echo_n "(cached) " >&6
 else
-  if test -n "$SVNVERSION"; then
-  ac_cv_prog_SVNVERSION="$SVNVERSION" # Let the user override the test.
+  if test -n "$HAS_GIT"; then
+  ac_cv_prog_HAS_GIT="$HAS_GIT" # Let the user override the test.
 else
 as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
 for as_dir in $PATH
@@ -5706,7 +5710,7 @@
   test -z "$as_dir" && as_dir=.
     for ac_exec_ext in '' $ac_executable_extensions; do
   if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_prog_SVNVERSION="found"
+    ac_cv_prog_HAS_GIT="found"
     $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
     break 2
   fi
@@ -5714,76 +5718,31 @@
   done
 IFS=$as_save_IFS
 
-  test -z "$ac_cv_prog_SVNVERSION" && ac_cv_prog_SVNVERSION="not-found"
+  test -z "$ac_cv_prog_HAS_GIT" && ac_cv_prog_HAS_GIT="not-found"
 fi
 fi
-SVNVERSION=$ac_cv_prog_SVNVERSION
-if test -n "$SVNVERSION"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $SVNVERSION" >&5
-$as_echo "$SVNVERSION" >&6; }
+HAS_GIT=$ac_cv_prog_HAS_GIT
+if test -n "$HAS_GIT"; then
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $HAS_GIT" >&5
+$as_echo "$HAS_GIT" >&6; }
 else
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
 fi
 
 
-if test $SVNVERSION = found
+else
+HAS_GIT=no-repository
+fi
+if test $HAS_GIT = found
 then
-	SVNVERSION="svnversion \$(srcdir)"
+    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"
 else
-	SVNVERSION="echo Unversioned directory"
-fi
-
-
-
-
-# Extract the first word of "hg", so it can be a program name with args.
-set dummy hg; ac_word=$2
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
-$as_echo_n "checking for $ac_word... " >&6; }
-if ${ac_cv_prog_HAS_HG+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  if test -n "$HAS_HG"; then
-  ac_cv_prog_HAS_HG="$HAS_HG" # Let the user override the test.
-else
-as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
-for as_dir in $PATH
-do
-  IFS=$as_save_IFS
-  test -z "$as_dir" && as_dir=.
-    for ac_exec_ext in '' $ac_executable_extensions; do
-  if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_prog_HAS_HG="found"
-    $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
-    break 2
-  fi
-done
-  done
-IFS=$as_save_IFS
-
-  test -z "$ac_cv_prog_HAS_HG" && ac_cv_prog_HAS_HG="not-found"
-fi
-fi
-HAS_HG=$ac_cv_prog_HAS_HG
-if test -n "$HAS_HG"; then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $HAS_HG" >&5
-$as_echo "$HAS_HG" >&6; }
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-
-if test $HAS_HG = found
-then
-    HGVERSION="hg id -i \$(srcdir)"
-    HGTAG="hg id -t \$(srcdir)"
-    HGBRANCH="hg id -b \$(srcdir)"
-else
-    HGVERSION=""
-    HGTAG=""
-    HGBRANCH=""
+    GITVERSION=""
+    GITTAG=""
+    GITBRANCH=""
 fi
 
 case $MACHDEP in