git-prompt.sh incompatible with non-basic global grep.patternType

Hi, I wanted to report something interesting that I found while tracing a severe slowdown in git-prompt.sh.

https://github.com/git/git/commit/6d158cba282f22fa1548af1188f78042fed30aed#diff-f37c4f4a898819f0ca4b5ff69e81d4d9R141

Way back in this commit, someone added a useful chunk of code that works perfectly with svn+ssh:// URLs under basic regexes:

+ local svn_upstream=($(git log --first-parent -1 \
+ --grep="^git-svn-id: \(${svn_url_pattern:2}\)" 2>/dev/null))

However, if I switch over to Perl regexes (or Extended):

git config --global grep.patternType perl

Then the command runs for one wall clock second and shows incorrect results on my repository. I eventually traced this to an issue with the regular expression provided, assuming the svn repository url is "svn+ssh://...":

git log ... --grep="^git-svn-id: \(svn+ssh://...\)" 2>/dev/null

The + sign isn't escaped in git-prompt.sh, which under non-basic regexes causes the match to fail entirely.

 - R.

ps. git log --basic-regexp does not fix the issue, as for unknown reasons (I'll start another thread) the command-line option doesn't override grep.patternType.