[SPARK-11169] Remove the extra spaces in merge script

Our merge script now turns
```
[SPARK-1234][SPARK-1235][SPARK-1236][SQL] description
```
into
```
[SPARK-1234] [SPARK-1235] [SPARK-1236] [SQL] description
```
The extra spaces are more annoying in git since the first line of a git commit is supposed to be very short.

Doctest passes with the following command:
```
python -m doctest merge_spark_pr.py
```

Author: Reynold Xin <rxin@databricks.com>

Closes #9156 from rxin/SPARK-11169.

dev/merge_spark_pr.py

@@ -300,24 +300,24 @@ def resolve_jira_issues(title, merge_branches, comment):
 def standardize_jira_ref(text):
     """
     Standardize the [SPARK-XXXXX] [MODULE] prefix
-    Converts "[SPARK-XXX][mllib] Issue", "[MLLib] SPARK-XXX. Issue" or "SPARK XXX [MLLIB]: Issue" to "[SPARK-XXX] [MLLIB] Issue"
+    Converts "[SPARK-XXX][mllib] Issue", "[MLLib] SPARK-XXX. Issue" or "SPARK XXX [MLLIB]: Issue" to "[SPARK-XXX][MLLIB] Issue"
 
     >>> standardize_jira_ref("[SPARK-5821] [SQL] ParquetRelation2 CTAS should check if delete is successful")
-    '[SPARK-5821] [SQL] ParquetRelation2 CTAS should check if delete is successful'
+    '[SPARK-5821][SQL] ParquetRelation2 CTAS should check if delete is successful'
     >>> standardize_jira_ref("[SPARK-4123][Project Infra][WIP]: Show new dependencies added in pull requests")
-    '[SPARK-4123] [PROJECT INFRA] [WIP] Show new dependencies added in pull requests'
+    '[SPARK-4123][PROJECT INFRA][WIP] Show new dependencies added in pull requests'
     >>> standardize_jira_ref("[MLlib] Spark  5954: Top by key")
-    '[SPARK-5954] [MLLIB] Top by key'
+    '[SPARK-5954][MLLIB] Top by key'
     >>> standardize_jira_ref("[SPARK-979] a LRU scheduler for load balancing in TaskSchedulerImpl")
     '[SPARK-979] a LRU scheduler for load balancing in TaskSchedulerImpl'
     >>> standardize_jira_ref("SPARK-1094 Support MiMa for reporting binary compatibility accross versions.")
     '[SPARK-1094] Support MiMa for reporting binary compatibility accross versions.'
     >>> standardize_jira_ref("[WIP]  [SPARK-1146] Vagrant support for Spark")
-    '[SPARK-1146] [WIP] Vagrant support for Spark'
+    '[SPARK-1146][WIP] Vagrant support for Spark'
     >>> standardize_jira_ref("SPARK-1032. If Yarn app fails before registering, app master stays aroun...")
     '[SPARK-1032] If Yarn app fails before registering, app master stays aroun...'
     >>> standardize_jira_ref("[SPARK-6250][SPARK-6146][SPARK-5911][SQL] Types are now reserved words in DDL parser.")
-    '[SPARK-6250] [SPARK-6146] [SPARK-5911] [SQL] Types are now reserved words in DDL parser.'
+    '[SPARK-6250][SPARK-6146][SPARK-5911][SQL] Types are now reserved words in DDL parser.'
     >>> standardize_jira_ref("Additional information for users building from source code")
     'Additional information for users building from source code'
     """
@@ -325,7 +325,7 @@ def standardize_jira_ref(text):
     components = []
 
     # If the string is compliant, no need to process any further
-    if (re.search(r'^\[SPARK-[0-9]{3,6}\] (\[[A-Z0-9_\s,]+\] )+\S+', text)):
+    if (re.search(r'^\[SPARK-[0-9]{3,6}\](\[[A-Z0-9_\s,]+\] )+\S+', text)):
         return text
 
     # Extract JIRA ref(s):
@@ -348,7 +348,7 @@ def standardize_jira_ref(text):
         text = pattern.search(text).groups()[0]
 
     # Assemble full text (JIRA ref(s), module(s), remaining text)
-    clean_text = ' '.join(jira_refs).strip() + " " + ' '.join(components).strip() + " " + text.strip()
+    clean_text = ''.join(jira_refs).strip() + ''.join(components).strip() + " " + text.strip()
 
     # Replace multiple spaces with a single space, e.g. if no jira refs and/or components were included
     clean_text = re.sub(r'\s+', ' ', clean_text.strip())