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.
diff --git a/runtime/doc/channel.txt b/runtime/doc/channel.txt
index fbe4d3f..111a56d 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -618,6 +618,9 @@
 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
diff --git a/src/logfile.c b/src/logfile.c
index 6c159b0..673468e 100644
--- a/src/logfile.c
+++ b/src/logfile.c
@@ -177,7 +177,9 @@
 	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);
 }
 
 /*
diff --git a/src/testdir/test_channel.vim b/src/testdir/test_channel.vim
index a99497d..438db8e 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -2293,7 +2293,8 @@
   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:')
diff --git a/src/version.c b/src/version.c
index 537f6aa..a04419a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    979,
+/**/
     978,
 /**/
     977,