build: log the start time of the current build session

Since `fdroid build --all` can run a long time, knowing when that command
was started will be very useful information for figuring out what the build
server is doing.

fdroidserver/build.py

@@ -288,6 +288,10 @@ def force_gradle_build_tools(build_dir, build_tools):
                                path)
 
 
+def _get_build_timestamp():
+    return time.strftime("%Y-%m-%d %H:%M:%SZ", time.gmtime())
+
+
 def transform_first_char(string, method):
     """Uses method() on the first character of string."""
     if len(string) == 0:
@@ -1055,6 +1059,7 @@ def parse_commandline():
 options = None
 config = None
 buildserverid = None
+starttime = _get_build_timestamp()
 
 
 def main():
@@ -1174,6 +1179,7 @@ def main():
 
         for build in app.builds:
             wikilog = None
+            build_starttime = _get_build_timestamp()
             tools_version_log = ''
             if not options.onserver:
                 tools_version_log = get_android_tools_version_log(build.ndk_path())
@@ -1270,7 +1276,7 @@ def main():
                     f.write('versionCode: %s\nversionName: %s\ncommit: %s\n' %
                             (build.versionCode, build.versionName, build.commit))
                     f.write('Build completed at '
-                            + time.strftime("%Y-%m-%d %H:%M:%SZ", time.gmtime()) + '\n')
+                            + _get_build_timestamp() + '\n')
                     f.write('\n' + tools_version_log + '\n')
                     f.write(str(e))
                 logging.error("Could not build app %s: %s" % (appid, e))
@@ -1295,7 +1301,9 @@ def main():
                     newpage = site.Pages[lastbuildpage]
                     with open(os.path.join('tmp', 'fdroidserverid')) as fp:
                         fdroidserverid = fp.read().rstrip()
-                    txt = "* build completed at " + time.strftime("%Y-%m-%d %H:%M:%SZ", time.gmtime()) + '\n' \
+                    txt = "* build session started at " + starttime + '\n' \
+                          + "* this build started at " + build_starttime + '\n' \
+                          + "* this build completed at " + _get_build_timestamp() + '\n' \
                           + '* fdroidserverid: [https://gitlab.com/fdroid/fdroidserver/commit/' \
                           + fdroidserverid + ' ' + fdroidserverid + ']\n\n'
                     if options.onserver: