Merge branch 'ps/stash-in-c'

"git stash" rewritten in C.

* ps/stash-in-c: (28 commits)
  tests: add a special setup where stash.useBuiltin is off
  stash: optionally use the scripted version again
  stash: add back the original, scripted `git stash`
  stash: convert `stash--helper.c` into `stash.c`
  stash: replace all `write-tree` child processes with API calls
  stash: optimize `get_untracked_files()` and `check_changes()`
  stash: convert save to builtin
  stash: make push -q quiet
  stash: convert push to builtin
  stash: convert create to builtin
  stash: convert store to builtin
  stash: convert show to builtin
  stash: convert list to builtin
  stash: convert pop to builtin
  stash: convert branch to builtin
  stash: convert drop and clear to builtin
  stash: convert apply to builtin
  stash: mention options in `show` synopsis
  stash: add tests for `git stash show` config
  stash: rename test cases to be more descriptive
  ...

strbuf.h

@@ -244,6 +244,15 @@ void strbuf_addchars(struct strbuf *sb, int c, size_t n);
  */
 void strbuf_insert(struct strbuf *sb, size_t pos, const void *, size_t);
 
+/**
+ * Insert data to the given position of the buffer giving a printf format
+ * string. The contents will be shifted, not overwritten.
+ */
+void strbuf_vinsertf(struct strbuf *sb, size_t pos, const char *fmt,
+		     va_list ap);
+
+void strbuf_insertf(struct strbuf *sb, size_t pos, const char *fmt, ...);
+
 /**
  * Remove given amount of data from a given position of the buffer.
  */
@@ -288,6 +297,13 @@ static inline void strbuf_addstr(struct strbuf *sb, const char *s)
  */
 void strbuf_addbuf(struct strbuf *sb, const struct strbuf *sb2);
 
+/**
+ * Join the arguments into a buffer. `delim` is put between every
+ * two arguments.
+ */
+const char *strbuf_join_argv(struct strbuf *buf, int argc,
+			     const char **argv, char delim);
+
 /**
  * This function can be used to expand a format string containing
  * placeholders. To that end, it parses the string and calls the specified