fetch: reduce duplicate in ref update status lines with placeholder

In the "remote -> local" line, if either ref is a substring of the
other, the common part in the other string is replaced with "*". For
example

    abc                -> origin/abc
    refs/pull/123/head -> pull/123

become

    abc         -> origin/*
    refs/*/head -> pull/123

Activated with fetch.output=compact.

For the record, this output is not perfect. A single giant ref can
push all refs very far to the right and likely be wrapped around. We
may have a few options:

 - exclude these long lines smarter

 - break the line after "->", exclude it from column width calculation

 - implement a new format, { -> origin/}foo, which makes the problem
   go away at the cost of a bit harder to read

 - reverse all the arrows so we have "* <- looong-ref", again still
   hard to read.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1220,6 +1220,11 @@ fetch.prune::
 	If true, fetch will automatically behave as if the `--prune`
 	option was given on the command line.  See also `remote.<name>.prune`.
 
+fetch.output::
+	Control how ref update status is printed. Valid values are
+	`full` and `compact`. Default value is `full`. See section
+	OUTPUT in linkgit:git-fetch[1] for detail.
+
 format.attach::
 	Enable multipart/mixed attachments as the default for
 	'format-patch'.  The value can also be a double quoted string

Documentation/git-fetch.txt

@@ -116,6 +116,11 @@ representing the status of a single ref. Each line is of the form:
 The status of up-to-date refs is shown only if the --verbose option is
 used.
 
+In compact output mode, specified with configuration variable
+fetch.output, if either entire `<from>` or `<to>` is found in the
+other string, it will be substituted with `*` in the other string. For
+example, `master -> origin/master` becomes `master -> origin/*`.
+
 flag::
 	A single character indicating the status of the ref:
 (space);; for a successfully fetched fast-forward;