transport.c: introduce core.alternateRefsPrefixes

The recently-introduced "core.alternateRefsCommand" allows callers to
specify with high flexibility the tips that they wish to advertise from
alternates. This flexibility comes at the cost of some inconvenience
when the caller only wishes to limit the advertisement to one or more
prefixes.

For example, to advertise only tags, a caller using
'core.alternateRefsCommand' would have to do:

  $ git config core.alternateRefsCommand ' \
      f() { git -C "$1" for-each-ref \
              refs/tags --format="%(objectname)" }; f "$@"'

The above is cumbersome to write, so let's introduce a
"core.alternateRefsPrefixes" to address this common case. Instead, the
caller can run:

  $ git config core.alternateRefsPrefixes 'refs/tags'

Which will behave identically to the longer example using
"core.alternateRefsCommand".

Since the value of "core.alternateRefsPrefixes" is appended to 'git
for-each-ref' and then executed, include a "--" before taking the
configured value to avoid misinterpreting arguments as flags to 'git
for-each-ref'.

In the case that the caller wishes to specify multiple prefixes, they
may separate them by whitespace. If "core.alternateRefsCommand" is set,
it will take precedence over "core.alternateRefsPrefixes".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 00224dea61..3486f9f749 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -627,6 +627,13 @@ Note that you cannot generally put `git for-each-ref` directly into the config
 value, as it does not take a repository path as an argument (but you can wrap
 the command above in a shell script).
 
+core.alternateRefsPrefixes::
+	When listing references from an alternate, list only references that begin
+	with the given prefix. Prefixes match as if they were given as arguments to
+	linkgit:git-for-each-ref[1]. To list multiple prefixes, separate them with
+	whitespace. If `core.alternateRefsCommand` is set, setting
+	`core.alternateRefsPrefixes` has no effect.
+
 core.bare::
 	If true this repository is assumed to be 'bare' and has no
 	working directory associated with it.  If this is the case a