.gitconfig: Improve mpr command

Change the `mpr` command so that it merges the specified pull request into the current branch¹, or if a branch name is specified², into the specified branch.

¹ git mpr <number>
² git mpr <number> <branch_name>

Closes #607.

.gitconfig

@@ -60,12 +60,16 @@
 	# List contributors with number of commits
 	contributors = shortlog --summary --numbered
 
-	# Merge GitHub pull request on top of the `master` branch
+	# Merge GitHub pull request on top of the current branch or,
+	# if a branch name is specified, on top of the specified branch
 	mpr = "!f() { \
+		declare currentBranch=\"$(git symbolic-ref --short HEAD)\"; \
+		declare branch=\"${2:-$currentBranch}\"; \
 		if [ $(printf \"%s\" \"$1\" | grep '^[0-9]\\+$' > /dev/null; printf $?) -eq 0 ]; then \
 			git fetch origin refs/pull/$1/head:pr/$1 && \
-			git rebase master pr/$1 && \
-			git checkout master && \
+			git checkout -B $branch && \
+			git rebase $branch pr/$1 && \
+			git checkout -B $branch && \
 			git merge pr/$1 && \
 			git branch -D pr/$1 && \
 			git commit --amend -m \"$(git log -1 --pretty=%B)\n\nCloses #$1.\"; \