patch 9.0.0979: ch_log() text can be hard to find in the log file

Problem:    ch_log() text can be hard to find in the log file.
Solution:   Prepend "ch_log()" to the text.

runtime/doc/channel.txt

@@ -618,6 +618,9 @@ ch_info({handle})						*ch_info()*
 ch_log({msg} [, {handle}])					*ch_log()*
 		Write String {msg} in the channel log file, if it was opened
 		with |ch_logfile()|.
+		The text "ch_log():" is prepended to the message to make clear
+		it came from this function call and make it easier to find in
+		the log file.
 		When {handle} is passed the channel number is used for the
 		message.
 		{handle} can be a Channel or a Job that has a Channel.  The

src/logfile.c

@@ -177,7 +177,9 @@ f_ch_log(typval_T *argvars, typval_T *rettv UNUSED)
 	channel = get_channel_arg(&argvars[1], FALSE, FALSE, 0);
 #endif
 
-    ch_log(channel, "%s", msg);
+    // Prepend "ch_log()" to make it easier to find these entries in the
+    // logfile.
+    ch_log(channel, "ch_log(): %s", msg);
 }
 
 /*

src/testdir/test_channel.vim

@@ -2293,7 +2293,8 @@ func Test_zz_ch_log()
   call ch_log('%s%s')
   call ch_logfile('')
   let text = readfile('Xlog')
-  call assert_match("hello there", text[1])
+  call assert_match("start log session", text[0])
+  call assert_match("ch_log(): hello there", text[1])
   call assert_match("%s%s", text[2])
   call mkdir("Xchlogdir1", 'D')
   call assert_fails("call ch_logfile('Xchlogdir1')", 'E484:')

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    979,
 /**/
     978,
 /**/