bugreport: include user interactive shell

It's possible a user may complain about the way that Git interacts with
their interactive shell, e.g. autocompletion or shell prompt. In that
case, it's useful for us to know which shell they're using
interactively.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bugreport.txt

@@ -29,6 +29,7 @@ The following information is captured automatically:
  - uname sysname, release, version, and machine strings
  - Compiler-specific info string
  - A list of enabled hooks
+ - $SHELL
 
 This tool is invoked via the typical Git setup process, which means that in some
 cases, it might not be able to launch - for example, if a relevant config file

bugreport.c

@@ -9,6 +9,7 @@
 static void get_system_info(struct strbuf *sys_info)
 {
 	struct utsname uname_info;
+	char *shell = NULL;
 
 	/* get git version from native cmd */
 	strbuf_addstr(sys_info, _("git version:\n"));
@@ -29,8 +30,13 @@ static void get_system_info(struct strbuf *sys_info)
 
 	strbuf_addstr(sys_info, _("compiler info: "));
 	get_compiler_info(sys_info);
+
 	strbuf_addstr(sys_info, _("libc info: "));
 	get_libc_info(sys_info);
+
+	shell = getenv("SHELL");
+	strbuf_addf(sys_info, "$SHELL (typically, interactive shell): %s\n",
+		    shell ? shell : "<unset>");
 }
 
 static void get_populated_hooks(struct strbuf *hook_info, int nongit)