Fix resource leak in git_log utility module

Resolve a resource leak I encountered in credits_git_gen.py during
development, where it could exit with an OSError: "Too many open files".

tools/utils/git_log.py

@@ -55,11 +55,11 @@ class GitCommit:
         ) + args
         # print(" ".join(cmd))
 
-        p = subprocess.Popen(
+        with subprocess.Popen(
             cmd,
             stdout=subprocess.PIPE,
-        )
-        return p.stdout.read()
+        ) as p:
+            return p.stdout.read()
 
     @property
     def sha1_short(self):
@@ -71,11 +71,11 @@ class GitCommit:
             "--short",
             self.sha1,
         )
-        p = subprocess.Popen(
+        with subprocess.Popen(
             cmd,
             stdout=subprocess.PIPE,
-        )
-        return p.stdout.read().strip().decode('ascii')
+        ) as p:
+            return p.stdout.read().strip().decode('ascii')
 
     @property
     def author(self):