contrib/credential/netrc: make PERL_PATH configurable

The shebang path for the Perl interpreter in git-credential-netrc was
hardcoded. However, some users may have it located at a different
location and thus, would have had to manually edit the script.

Add a .perl prefix to the script to denote it as a template and ignore
the generated version. Augment the Makefile so that it generates
git-credential-netrc from git-credential-netrc.perl, just like other
Perl scripts.

The Makefile recipes were shamelessly stolen from
contrib/mw-to-git/Makefile.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/credential/netrc/.gitignore

@@ -0,0 +1 @@
+git-credential-netrc

contrib/credential/netrc/Makefile

@@ -1,8 +1,30 @@
 # The default target of this Makefile is...
 all::
 
-test:
+SCRIPT_PERL = git-credential-netrc.perl
+GIT_ROOT_DIR = ../../..
+HERE = contrib/credential/netrc
+
+SCRIPT_PERL_FULL = $(patsubst %,$(HERE)/%,$(SCRIPT_PERL))
+
+all:: build
+
+build:
+	$(MAKE) -C $(GIT_ROOT_DIR) SCRIPT_PERL="$(SCRIPT_PERL_FULL)" \
+                build-perl-script
+
+install: build
+	$(MAKE) -C $(GIT_ROOT_DIR) SCRIPT_PERL="$(SCRIPT_PERL_FULL)" \
+                install-perl-script
+
+clean:
+	$(MAKE) -C $(GIT_ROOT_DIR) SCRIPT_PERL="$(SCRIPT_PERL_FULL)" \
+                clean-perl-script
+
+test: build
 	./t-git-credential-netrc.sh
 
-testverbose:
+testverbose: build
 	./t-git-credential-netrc.sh -d -v
+
+.PHONY: all build install clean test testverbose