The error is because Git bash/GUI uses an old version of BASH.
This should fix it -

diff --git a/commit-msg b/commit-msg
index fd6b1ce..3995eac 100755
--- a/commit-msg
+++ b/commit-msg
@@ -70,9 +70,12 @@ msg_first() {
printErrorAndExit 'The first line may not have leading or trailing space:
['"$line"']'
fi

• if [[ ( "$(echo "$line" | cut -c1-4)" =~ (BUG:|ENH:)$ ) ||
• ( "$(echo "$line" | cut -c1-5)" =~ (COMP:|PERF:)$ ) ||
• ( "$(echo "$line" | cut -c1-6)" =~ (STYLE:)$ ) ]]; then
• firstSixLetters=$(echo "$line" | cut -c1-6)
• if [ "${firstSixLetters:0:4}" == "BUG:" -o \
• "${firstSixLetters:0:4}" == "ENH:" -o \
• "${firstSixLetters:0:5}" == "COMP:" -o \
• "${firstSixLetters:0:5}" == "PERF:" -o \
• "$firstSixLetters" == "STYLE:" ]; then

  first line okay

  state=second
  else

And then, incrementing SetupForDevelopment_VERSION would notify developers to update their version of hooks.

Thanks Sankhesh.

@Johan: Could you locally try the change suggested by Sankhesh ?

It works ! Thanks Sankhesh !

Fixed in r21946
See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=21946

Closing resolved issues that have not been updated in more than 3 months

Fix committed to 2145-support-for-installing-extension-from-file branch.