Refactor git_set_argv0_path() to git_extract_argv0_path()

This commit moves the code that computes the dirname of argv[0]
from git.c's main() to git_set_argv0_path() and renames the function
to git_extract_argv0_path().  This makes the code in git.c's main
less cluttered, and we can use the dirname computation from other
main() functions too.

[ spr:
 - split Steve's original commit and wrote new commit message.
 - Integrated Johannes Schindelin's
   cca1704897 while rebasing onto master.
]

Signed-off-by: Steve Haslam <shaslam@lastminute.com>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec_cmd.c

@@ -20,9 +20,19 @@ const char *system_path(const char *path)
 	return path;
 }
 
-void git_set_argv0_path(const char *path)
+const char *git_extract_argv0_path(const char *argv0)
 {
-	argv0_path = path;
+	const char *slash = argv0 + strlen(argv0);
+
+	while (argv0 <= slash && !is_dir_sep(*slash))
+		slash--;
+
+	if (slash >= argv0) {
+		argv0_path = xstrndup(argv0, slash - argv0);
+		return slash + 1;
+	}
+
+	return argv0;
 }
 
 void git_set_argv_exec_path(const char *exec_path)

exec_cmd.h

@@ -2,8 +2,8 @@
 #define GIT_EXEC_CMD_H
 
 extern void git_set_argv_exec_path(const char *exec_path);
-extern void git_set_argv0_path(const char *path);
-extern const char* git_exec_path(void);
+extern const char *git_extract_argv0_path(const char *path);
+extern const char *git_exec_path(void);
 extern void setup_path(void);
 extern const char **prepare_git_cmd(const char **argv);
 extern int execv_git_cmd(const char **argv); /* NULL terminated */

git.c

@@ -442,21 +442,12 @@ static int run_argv(int *argcp, const char ***argv)
 
 int main(int argc, const char **argv)
 {
-	const char *cmd = argv[0] && *argv[0] ? argv[0] : "git-help";
-	char *slash = (char *)cmd + strlen(cmd);
+	const char *cmd;
 
-	/*
-	 * Take the basename of argv[0] as the command
-	 * name, and the dirname as the default exec_path
-	 * if we don't have anything better.
-	 */
-	while (cmd <= slash && !is_dir_sep(*slash))
-		slash--;
-	if (cmd <= slash) {
-		*slash++ = 0;
-		git_set_argv0_path(cmd);
-		cmd = slash;
-	}
+	if (argv[0] && *argv[0])
+		cmd = git_extract_argv0_path(argv[0]);
+	else
+		cmd = "git-help";
 
 	/*
 	 * "git-xxxx" is the same as "git xxxx", but we obviously: