create-pull-request: set subject automatically for cover latter

Set cover letter's subject automatically as the patch's subject when
there is only one patch.

[YOCTO #9410]

(From OE-Core rev: 162b80f8a4670befaf6ffd2c178671cf7370b767)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/create-pull-request

@@ -261,7 +261,13 @@ if [ -n "$BODY" ]; then
 	sed -i "/BLURB HERE/ d" "$CL"
 fi
 
-# If the user specified a subject, replace the SUBJECT token with it.
+# Set subject automatically if there is only one patch
+patch_cnt=`git log --pretty=oneline ${RELATIVE_TO}..${L_BRANCH} | wc -l`
+if [ -z "$SUBJECT" -a $patch_cnt -eq 1 ]; then
+    SUBJECT="`git log --format=%s ${RELATIVE_TO}..${L_BRANCH}`"
+fi
+
+# Replace the SUBJECT token with it.
 if [ -n "$SUBJECT" ]; then
 	sed -i -e "s/\*\*\* SUBJECT HERE \*\*\*/$SUBJECT/" "$CL"
 fi