use ps -f and awk to emulate pgrep

There is no pgrep in git for windows

bin/git-changelog

@@ -317,7 +317,20 @@ _exit() {
   stty sane; echo; echo "caught signal, shutting down"
 
   IFS=$'\n'
-    pid_list=( $(pgrep -P $$) )
+  # The format of `ps` is different between Windows and other platforms,
+  # so we need to calculate the total column number(COL_NUM) of header first.
+  # Why don't we just use the last column?
+  # Because the body of CMD column may contain space and be treated as multiple fileds.
+  pid_list=( $(ps -f | 
+    awk -v ppid=$$ 'NR == 1 {
+      COL_NUM = NF
+    }
+    $3 == ppid {
+      # filter out temp processes created in this subshell
+      if ($COL_NUM != "ps" && $COL_NUM != "awk" && $COL_NUM !~ "bash$")
+        print $2
+    }')
+  )
   IFS="$defaultIFS"
 
   local _pid